The Weaver and the Cowherd

The Weaver and the Cowherd

A journey to places of historical interest.

Like many explorers in Elite Dangerous, I've got an interest in astronomy. One of the things that drew me to the game was the prospect of having a full-scale galaxy to roam around in, with many features taken directly from modern astronomical data. Like many explorers, I get a thrill out of learning the astronomy behind many of the places I go to. Because of this, I decided to visit a number of places that are historically significant to galactic astronomy, and write them up in a series of posts. Many of these places are relatively close by, making them very suitable for first-time explorers to visit in inexpensive ships. I'm calling this an expedition, but it's more of a series of small trips bound together with a common theme.

I hope you enjoy the journey as much as I am.

If you'd like to learn more about any of the places we visit, I recommend Ken Croswell's The Alchemy of the Heavens as an interesting and accessible history of galactic astronomy.

The immediate neighborhood of the sun

Part I - Altair and Vega, The Weaver and the Cowherd

The name of this expedition comes from a Chinese folk tale. Zhinu was the daughter of the Emperor of Heaven. Coming to earth to bathe, she met Niulang, a handsome but poor cowherd, and fell in love with him. They married, and she bore him two children. When the Emperor of Heaven discovered that his daughter was missing, he sent the girl's grandmother to bring her back.

Seeing his wife taken from him, the cowherder used the hide of a magical ox to fly after her along with their two children. The grandmother, because of her age, could not travel fast, used her hairpin to scratch a river full of pinpricks to stop the cowherd from following them.

The river of pinpricks became the milky way, and so divides the two lovers for all eternity. Zhinu shone on one side of the river as the star Vega, while Niulang watched her from the other side as the star Altair. Their children clung close to their father as Beta and Gamma Aquilae.

Vega requires a permit from the Federation, which I don't have, and so I had to be satisfied with a visit to the star Altair.

Altair is a bright star of type A7. Barely 16 light years from Sol, it's an easy trip for any ship. Young stars rotate rapidly, slowing down as they age, and Altair betrays it's youth in this way. Merely 630 million years old, Altair rotates once every 8.9 hours, fast enough to visibly flatten it from a sphere.

Altair is especially interesting because it is one of the few stars that have been directly imaged.

Back to Zhinu and Niulang, however. Tales of their love became well known, and the Emperor of the Heavens was persuaded by the magpies to allow the lovers a small mercy: Once a year the magpies would be allowed to form a bridge over the river and allow them to reunite and see each other, if only for a day.

Next: Measuring the heavens - Parallax and 61 Cygni
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Sounds like a great adventure Cmdr. I love the real image of the "flattened" star...hopefully one day FD will replicate this phenomenon, and then you will have to return again.

Good luck with your future trips and I look forward to reading about them

Part II - Measuring the Heavens

Parallax and 61 Cygni

Over the years, many people have tried to find the distances to the stars. Robert Hooke outlined in 1674 the problems of looking for annular motion of stars and Isaac Newton tried to calculate the distance of Sirius by comparing its brightness to that of the Sun. Astronomer William Herschel (who also discovered the planet Uranus) spent a great deal of his life trying to assemble a map of stellar distribution, including distances, based on the assumption that every star has the same brightness (an assumption that was known to be false at the time, since astronomers had already catalogued binary stars of different brightnesses).

The first successful solution used the same technique that we use naturally in order to perceive depth: parallax. When we look at something from two different points, it appears to shift in relation to things in its background. With our two eyes, we're able to view the same objects at the same time, deducing distance based on this principle.

Measuring stellar distances uses basically the same technique, rendered complex by the fact that stellar parallax is so small that it remained unobservable for most of human history. Reckoned against early geocentric views of the universe, this was easily explained: if the earth is the center of the universe, then it's position never changes and thus the angle to the stars never changes. Early astronomer Tycho Brahe used the apparent lack of observable parallax as an argument against Copericus' heliocentric universe, arguing that if the earth moved around the sun, the only way for there to not be observable stellar parallax - measured by comparing the position of a star in two observations 6 months apart - would be for the stars to be unimaginably far away. Once Copernicus was proven right, however, the idea that the stars are very distant was readily accepted, and attempts to observe stellar parallax began.

Which brings us to 61 Cygni. A dim binary pair, the star system was first reliably observed only in 1753 by astronomer James Bradley. William Herschel began to systematically observe it as part of a study on binary stars, hoping that he could use orbital dynamics to determine their distances by measuring their orbits and the time it took them to revolve around one another. Astronomer Giuseppe Piazzi, who also discovered the asteroid Ceres, compared observations of 61 Cygni with ones made by Bradley 40 years earlier and determined that it had a large apparent motion through the sky. Reasoning that it's rapid apparent motion meant that it was probably one of the closer stars, he suggested that it would be a prime candidate for measuring parallax.

61 Cygni showing proper motion at one year intervals.

Many attempts to measure the parallax of 61 Cygni followed. It was readily acknowledged that the parallax would be small - equivelant to measuring the apparent angle taken up by a US quarter at a distance of 3.5 kilometers. Early instruments were not up to the task, and rapid progress was made in an attempt to develop instruments which could accurately measure stellar parallax. Finally, in 1837 and 1838, Friedrich Wilhelm Bessel used a new and highly accurate instrument called a Fraunhofer heliometer to carry out a series of observations, establishing a stellar parallax of 61 Cygni of 313.6 milli arc seconds (or about 0.000087 degrees), giving it a distance of 10.4 light years away, and for the first time directly measuring the distance to another star other than the Sun. Parallax measurements using the heliometer soon followed for Vega and Alpha Centauri, and the the first rung on the yardstick of the universe had been established.

In game, 61 Cygni A is a rather ordinary K-class star, mainly notable for Broglie Terminal, a well-equipped starport very close to the primary.

Next: Galactic rotation - Jacobus Kapteyn, the astronomer without a telescope.
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Love these little astronomy/history lessons...keep them coming.

I am, however, intrigued by the fact Friedrich Wilhelm Bessel used his Fraunhofer heliometer one hundred and one years apart, first in 1837 and then in 1938. He must have been very long-lived... ;)

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Part III - Galactic Rotation

Jacobus Kapteyn, the astronomer without a telescope.

One of the great names in astronomy is the Dutch astronomer Jacobus Kapteyn, who spent the majority of his professional life as Professor of Astronomy at the University of Groningen. The interesting thing about that is Groningen had no observatory, despite Kapteyn's best efforts to raise funds to build one. To overcome this, he maintained contacts with astronomers around the world, volunteering to help catalog and analyze their data. In a sense, despite having no telescope of his own, Kapteyn had the world's observatories available to him. Astronomical photography was becoming common, and Kapteyn capitalized on this by specializing in large scale photographic analysis. The first fruits of this was the titanic Cape Photographic Durchmusterung, cataloging nearly 455,000 stars in the Southern Hemisphere.

Building on this work, he began to study the proper motion of stars - examining how stars moved through space. At the time it was thought that stars moved in random directions, but Kapteyn found that stars could be divided into two streams, moving in nearly opposite directions.

Kapteyn's illustration of his two Star Streams (1904). At only a selection of the regions for which he had data (10 of the available 28), he plots the distribution of proper motions on the sky. If there were no preferential streaming each distribution would be round, but Kapteyn found these distributions to be asymmetrical and to point to an apex in the middle of the figure. The systematic behaviour across the sky is evidence for two Star Streams. It was soon shown by K. Schwarzschild that the pattern could in an alternative, but much more physical manner be explained as an anisotropy in the distribution of the stellar velocities.

This turned out to be the first evidence of galactic rotation, with the streams being stars at opposite sides of the galaxy, moving in different relative directions as they orbit, an interpretation made by another prominent early 20th century astronomer, Karl Schwarzschild (who lent his name to the Schwarzchild radius, which describes the event horizon of a black hole). Kapteyn himself felt that the two streams were distinct populations of stars, supported by evidence at the time that the streams were spectrally different.

Stellar Spectrum and the HR Diagram

Physicists have been looking at the solar spectrum since Isaac Newton first used a prism to split sunlight into a rainbow spectrum. In the early 1800s, Joseph von Fraunhofer (who built the Fraunhofer heliometer that Bessel used to observe the parallax of 61 Cygni) used his skills as a glass maker to create very high quality prisms, which allowed him to observe dark lines in the spectrum of the sun. By mating his prisms to telescopes, he created the first astronomical spectroscope, observing dark lines in the spectra of other stars as well. These dark lines are the fingerprints of the stars: ions in the atmosphere of the star absorb light at certain wavelengths, leaving these so-called Fraunhofer (that guy gets around!) lines in the spectrum.

Early work on stellar spectra identified that stars could be grouped based on their spectra. These groups were gradually refined, becoming the stellar classification system we use today.

In the late 19th century, large-scale photographic spectroscopic surveys of stars were performed at Harvard, resulting in the Henry Draper Catalogue which is still used today. If you search for NGC 2374 Sector BA-A e0 in the galactic map, for instance, you'll find the star HD 55854 nearby - The "HD" in that name stands for Henry Draper. Two astronomers, Ejnar Hertzsprung and Henry Norris Russell, simultaneously created graphs plotting the stellar classifications of stars versus their luminosity. The result was unexpected. The vast majority of stars cluster along a curve through the middle of the graph, with a few branches splitting off from that.

This diagram is known as the HR Diagram (for Hertzsprung and Russell), and it remains one of the most powerful tools in the understanding of stellar evolution.

Now back to Kapteyn: At about the same time as Hertzprung and Russell were plotting their star diagrams, Kapteyn launched a plan for a major study of the distribution of stars in the Galaxy. Factoring apparent magnitude, spectral type, radial velocity, and proper motion of stars in 206 zones, it was the first coordinated statistical analysis in astronomy, using data from over forty observatories. Understanding that the HR diagram could be used to estimate distance, he ultimately created a map of the galaxy with this data.

Kapteyn postulated a lens-shaped galaxy in which the density of stars was high in the center, and gradually decreased out towards the edge. In his model the galaxy was thought to be 40,000 light years across, with the sun being relatively close (2000 light years) to the center. While this model was correct for many details of the galaxy, it severely underestimated the distances of stars in the plane of the galaxy, largely because Kapteyn underestimated the effects of gas and dust in the galaxy. These absorb light, dimming the stars and making them look more distant, and making the Kapteyn Universe too small, especially in the direction of the core. Still, it was a major advancement that allowed for initial studies into stellar dynamics.

One of the artifacts of Kapteyn's studies is his discovery, along with astonomer Robert Innes, that a nearby class M red dwarf possessed an extremely high proper motion. At the time of this discovery, it had the highest proper motion of any star known, dethrowing Groombridge 1830. It has since come to be known as Kapteyn's Star.

Kapteyn's Star is distinctive in a number of regards: It has a high velocity, over 245 km/s in relation to the sun; it orbits the milky way retrograde (backwards); and it is the nearest known halo star, meaning it did not form in the disk of the galaxy. It is a member of a moving group of stars that share a common trajectory through the galaxy, named the Kapteyn moving group. It's currently thought that these stars used to be members of the globular cluster Omega Centauri, which itself is thought to be the remnant of a dwarf galaxy that merged with the milky way, stripping away stars (including Kapteyn's Star) in the process.

Kapteyn's Star provides an interesting clue into how the mechanics of hyperspace jumps might work in Elite Dangerous, too. If you jump from one system to another and close the throttle when the jump countdown begins, you will emerge from the jump at the mimimum supercruise velocity of 30 km/s relative to the primary.

Kapteyn's Star, recall, has a velocity of just over 245 km/s in relation to the Sun, and lies only 12.76 light years from the Sun. If you outfit a ship with a frameshift drive capable of making the jump from the Sun to Kapteyn's Star, closing the throttle at the countdown, you'll see that you still emerge with a velocity of only 30 km/s relative to the primary. The other 215 km/s has somehow been absorbed as part of the jump process. There could be any number of reasons this could be happening (not the least of which is it simply makes the jump convenient), but it's interesting that it happens nonetheless.

Next: Standard Candles - Cepheid and RR Lyra variables, rungs in the ladder of stellar distances.
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Part IV - Standard Candles

Cepheid and RR Lyra variables, rungs in the ladder of stellar distances

Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.
- Douglas Adams, The Hitchhiker's Guide to the Galaxy

It's been known for a long time that the stars are very far away. One of the objections to Copernicus' heliocentric solar system, you'll recall, is that our inability to measure a stellar parallax (in the 16th century) would imply that the stars were simply too far away, should the earth revolve around the sun. Yet once Copernicus' theory was accepted, so was the corrolary of immense stellar distances.

Even before Bessel first measured the parallax of 61 Cygni, attempts were made to calculate stellar distances in other ways. William Herschel attempted in the late 18th century to develop a map of the milky way by counting stars in different directions. He assumed that all stars were roughly equal in brightness, which would imply that dimmer stars were farther away. While the may he developed was crude, he was able to identify that the stars in the galaxy mostly lie in a plane. Thus began modern astronomy's quest for a true standard candle - a type of star that was all the same brightness. It is both fortunate and ironic that the first real standard candles turned out to be stars which don't even remain the same brightness from day to day.

William Herschell's map of the galaxy, based on the observation of several hundred stars

In 1638, astronomer Johannes Holwarda noticed that the star Omicron Ceti changed it's brightness, pulsating in a cycle 11 months long. By 1786 ten variable stars were known, including a pair called Eta Aquilae and Delta Cephei, and the number of variable stars known has continuously increased since then. While many variable stars were known since the 18th century to be binaries, changing in brightness as one star eclipses the other, others changed brightness for different reasons. With the advent of photography, it was posible to reliably plot out the changes in a variable star's brightness, aiding research into the behavior of variable stars.

As part of this research, astronomer Henrietta Swan Leavitt was studying variable stars in the Magellanic Clouds in the late 19th and early 20th century. In 1908, she published a paper on her work, noting that a few of the variables showed a pattern: brighter ones appeared to have longer periods. Noting that these stars were all effectively at the same distance from Earth, she tracked their brightness and the length of their cycle and determined that there was a strong relationship between the two. Her discovery is known as the "period-luminosity relationship", and the stars that matched this pattern are of a type known as Cepheids, after the variable star Delta Cephei. Because it was easy to plot the period of a Cepheid variable's cycle, you could compare Cepheid variables and find the ratio of how far away they were. Once Cepheids were identified that were near enough that their distances could be established through parallax, they became useful as standard candles for establishing absolute distances.

As work on Cepheid variables continued, it became clear that there were several types of Cepheid variables, each with their own period-luminosity relationship. With each refinement, the ability to determine the distances to different objects increased. From 1915 to the 1930s, a class of variable stars called RR Lyraes (after their prototype star RR Lyrae) were discovered to obey their own period-luminosity relationship distinct from Cepheids, while in the 1940s astronomer Walter Baade recognized that there were two separate populations of Cepheid stars based on the amount of metals (elements other than hydrogen and helium) in their makeups.

While the different populations of Cepheid and RR Lyrae stars differ in mass and composition, they are alike in that they are all old. All of these stars have burned a good deal of their hydrogen into helium, and have left the main sequence, expanding into giants.

In 1915, Arthur Stanley Eddington proposed a theory that both Cepheid and RR Lyrae variables physically pulsed, expanding and contracting in order to generate their extremely consistent variations in brightness. In 1953, S. A. Zhevakin identified ionized helium as the likely catalyst for this pulsation, relying on the fact that doubly ionized helium (helium whose atoms are missing both electrons) is more opaque than single ionized helium. In this mechanism (called the Eddington valve, or Kappa-mechanism), helium is heated by the star until it becomes doubly ionized, growing more opaque and absorbing more of the star's radiation. This makes the star appear dim, while also heating the helium and making it expand. As it expands, it cools, and so becomes less ionized and more transparent, allowing the radiation to escape. This allows the star to brighten while the expansion stops and then reverses due to the star's gravitational attraction until the helium grows hot enough to doubly ionize, repeating the process again.

Delta Cephei - the prototype for Cepheid variables

Sitting at a distance of about 900 light years from Earth, Delta Cephei is the star that gives it's name to the class of Cepheid variables. Because it's a variable star, it's stellar class changes over time, varying from about F5 to G3 over a period of a little more than 5 days. For whatever reason, the galactic map and ship scanner calls it a class B star. Delta Cephei, like all classical Cepheids, is a high mass star that has left the main sequence, expanding into a yellow (or blue, in the game) giant.

RR Lyrae - the prototype for RR Lyrae variables

RR Lyrae are distinct from classical Cepheids in that they are low mass stars - less massive than the sun - with very trace amounts of elements other than hydrogen and helium. They are intrinsicly dimmer than classical Cepheids, but there are far more of them, just as there are far more red stars in the galaxy than hot blue ones. Like Cepheids, they follow a period-luminocity relationship, which makes them useful for establishing distances within the Milky Way and it's nearby neighboring galaxies. Stars of this class have consumed the hydrogen at their core, evolved away from the main sequence, and passed through the red giant stage. Energy is now being produced by the thermonuclear fusion of helium in the core, brightening the star.

RR Lyrae itself is a small blue star varying between class A8 and F7, with a period of 13 hours and 36 minutes. Sitting nearly 900 light years away in the constellation Lyrae. Like all RR Lyrae stars, it is an old star that formed during the early period of the Universe when there was a lower abundance of other elements in star-forming regions. It follows an extremely eccentric orbit through the galaxy, coming as close as 6800 light years to the center of the galaxy and ranging as far as 60,000 light years at its farthest point.

RR Lyrae appears in game with the designation HIP 95497.

Next: Stellar populations over the life of the universe.
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Part V - Stellar Populations

Demographics over the lifespan of the universe

The nearest major galaxy to our own is M31, the Andromeda Galaxy. It's near enough and big enough that on a clear, dark night you can see it with your naked eye as a faint, slightly fuzzy patch. Comet-hunter Charles Messier included it in his famous catalog of diffuse objects that could be mistaken for comets. As telescopes improved and photography came into widespread use, it's spiral nature became clear. At the time, the prevailing view of the cosmos was that the universe consisted solely of our galaxy, and the so-called Andromeda Nebula (along with all the other spiral "nebulae") were theorized to be solar systems being formed.

In 1917, astronomer Heber Curtis observed a nova within M31. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were some 10 magnitudes (about 10,000 times) fainter than those observed elsewhere in the sky, and correctly surmised that they were much further away. He estimated a distance of 500,000 light years as the distance between the Earth and M31, and became a proponent of the "island universe" hypothesis, which held that spiral nebulae were actually independent galaxies.

The great American astronomer Edwin Hubble siezed upon Henrietta Swan Leavitt's period-luminosity relationship in Cepheid variables as a way to settle the debate. Using the newly-completed 100 inch telescope at Mount Wilson Observatory, he was able to identify Cepheid variables in astronomical photographs of M31, confirming that it lay outside of the galaxy. This discovery, however, came with problems of its own. While blue supergiants could be identified in the spiral arms of M31, no stars could be resolved in the core of the galaxy, nor in its companion galaxies. It grew into a large enough problem that theories were advanced speculating that there were no stars in a galaxy's core, only glowing gas.

Building on Hubble's work was his colleague, German astronomer Walter Baade. Working at the Mount Wilson Observatory during World War II, Baade took advantage of wartime blackout conditions to resolve stars at the center of M31. At the time, most astronomical photographs were taken with plates sensitive to blue light. These were "faster", reducing the problems of managing the telescope during long exposures. Baade theorized that stars couldn't be resolved in the center of M31 on blue sensitive plates because there simply were no blue stars there. New fast red-sensitive photographic had recently been developed, and Baade made a series of observations of M31 using these plates, and succeeded in resolving the center of M31. His theory had been correct: there were no blue stars in the center of M31.

As Baade looked more closely at the stars he had resolved, he began to notice other interesting things about them. Most important of these distinctions was their chemical composition, called metallicity by astronomers partially because a star's iron spectra was commonly used as an index of the abundance of elements other than hydrogen and helium. Stellar spectra have been systematically gathered for stars in the observable galaxy since the late 19th century, and for the most part these observations have pointed to very similar compositions for most stars: About 73% hydrogen, 25% helium, and 2% "metals" by mass. The stars at the center of M31, on the other hand, had much lower amounts of metals. Baade began to consider these stars to be fundamentally different, and began to refer to them as Population II stars, to distinguish them from "ordinary" Population I stars.

But Population II stars didn't just exist in the cores of galaxies. The companion galaxies to M31 were made up of Population II stars, as were the globular clusters of our own galaxy, as well as our companion galaxies. Individual stars within the galactic halo also had extremely low metallicities.

Population II stars make up globular clusters, like M80 in Scorpius

Once this distinction was made, other common characteristics of Population II stars became apparent: They moved differently. Most stars in the galaxy orbit in the plane of the galaxy, moving around the galactic core in the same direction, all in a disc. Population II stars, on the other hand, moved in highly elliptical orbits and did not tend to remain in the plane of the galaxy.

It was known by this time that stars fused hydrogen into helium, and it was theorized that the heavier elements could be produced in them, but the prevailing theory was that most of the heavier elements had been created during the Big Bang. The existance of Population II stars, with their vastly reduced abundances of heavier elements, laid doubt on this theory, suggesting that most of the heavier elements in the universe were created after the Big Bang. This led to the theory that Population II stars were older, with their spectra representing conditions much earlier in the life of the universe, at the time the stars were formed.


Arcturus is a type K0 orange giant located only 36.7 light years from Earth in the constellation of Boötes. Arcturus is especially noteworthy for its large proper motion, or sideways motion across our sky. Only Alpha Centauri - our sun's nearest neighbor among the stars - has a higher proper motion among the first-magnitude, or bright, stars in the stellar neighborhood. This high proper motion tells us that Arcturus is moving at a tremendous rate of speed relative to our solar system: 122 km/s. Moreover, Arcturus is not moving with the general stream of stars in the flat disk of the Milky Way galaxy. Instead, it is cutting perpendicularly through the galactic disk. This, combined with it's low metallicity (17-32% that of the Sun), mark it as a Population II star, and old.

Arcturus lies off the main sequence of stars on the HR diagram, being significantly redder than most stars of its luminosity, which is an area we haven't touched on yet. But it will be a fantastic segue into...

Next: Stellar evolution and the age of stars.
Another excellent post, I'm reasonably familiar with the current state of astronomy but the history of how we got there is something I know a lot less about so that makes for a fascinating read. +1
Part VI - The Alchemy of the Heavens

Stellar evolution and the age of stars

Errata In Part V - Stellar Populations I erroneously stated that Walter Baade noted the low metallicity of Population II stars. It was Joseph W. Chamberlain and Lawrence H. Aller who first published research identifying low metallicity stars in 1951.

My vacation put a crimp in these things, but I hope to get back on track with them now.

In 1906, Karl Schwartzchild published a paper modeling a star as an incandescent, stable ball of gas in considerable detail, using basic principles of physics. This model showed that the density and temperature of the gas increased towards the core, and described how energy could be transported from the center of a star outwards by either convection (like water boiling in a pot) or radiation (like heat escaping a fireplace). Sir Arthur Eddington built on this to describe details of conditions in the center of a star. His model showed that a star would contract while its centers heated up until it reached about 20 million degrees, at which time it would stop contracting and become a stable star.

In 1939, astronomers Hans Bethe and Carl von Weizsacker developed study of nuclear fusion, showing how atoms could be transformed into other atoms by fusing with protons, releasing enough energy to power a star. This model, known as the carbon-nitrogen-oxygen, or CNO cycle, worked well for stars like our own. For less massive stars, however, with their lower core temperatures, the energy released by the CNO cycle could not predict the luminosity of the stars. Bethe determined that for these stars, free protons could bind to other free protons to create helium in quantities sufficient to explain the stars, in a cycle known as the proton-proton chain.

The first stellar models were all developed by hand, using desk calculators. This led to trade-offs between examining a crude model of the interior of a star over a long period of time, or using a detailed model but only being able to examine a brief amount of time. As the electronic computer became available, these were refined into detailed models that showed how the hydrogen was consumed in the cores of stars, leading from the main sequence to the so-called red giant phase.

In 1955, R. Harm and Martin Schwartzchild (son of Karl Schwartzchild - astronomy runs strong in that family!) published 15 models, some laboriously calculated by hand over the course of a year, some using electronic computers, of stellar evolution. The models presented the interior of the stars in three zones: the core, the outer envelope, and the intermediate zone between them. These models, though, simple, presented clear views of how stars change over time, and represented the first comprehensive attempt to understand how stars evolve.

This work ushered in a new era in astronomy where electronic computers were used to model conditions in stars, developing more and more detailed views into the lives of the denizens of the galaxy. Built up by gravity's inexorably attraction, and powered by the intense conditions that this has produced in their cores, stars can be regarded as finely tuned furnaces. The radiation and heat produced by fusion in the cores of the stars counters the inward pulling gravity, and each star settles (for the most part) into a stable state known as hydrostatic equilibrium, where the two forces balance out.

Stars of different mass perform their balancing acts in different ways. Low mass stars, under about 0.8 solar masses, are entirely convective: Gamma rays produced by their nuclear reactions are immediately absorbed by stellar material, heating it. This stellar material rises from the hot cores to the surface, cools, and then sinks back into the core. Because of this, fresh hydrogen is always available to feed the thermonuclear reactor at the core throughout their lives as stars. And their lives are long indeed - much longer than the universe has been in existence. Every low-mass star that the Milky Way has ever spawned is still around today.

More massive stars, ranging from about 0.8 up to 8 solar masses, differ from their less massive brethren in how the energy they produce gets transported to the surface. Because of the intensity of the nuclear reactions at their core, the stellar material absorbs the gamma rays from the core and reradiates it as lower energy X-rays. These are in turn absorbed by the next higher layer above the core, which warms and re-ratiates at an even lower ultraviolet energies, and so on up towards the surface of the star. The photons careen off of the charged particles around them in a sort of drunkards walk that has only a very slight outward tendency, compelled by the decreasing temperature as they move away from the core. Once the radiant energy is about two thirds of the way to the surface, roiling convection takes over, transporting heated gas to the surface where it cools and sinks back down again.

For stars on the more massive end of this range, the CNO process drives energy generation in their cores. Trace amounts of carbon, nitrogen, and oxygen catalyze the fusing of hydrogen into helium. This reaction requires higher central temperature and pressure. The payoff is a much greater reaction rate which drives convection in the lower layers. For stars like this, a deep convection zone surrounds the core, while above this zone, radiative processes transport the energy the rest of the way to the surface.

Life gets more dramatic for stars with even greater masses. The crushing weight of these stars compresses their cores to incredible pressures and temperatures, driving thermonuclear reactions into overdrive. At these rates, the sheer pressure of photons on the stellar material is enough to push on the overlying layers and destabilize them.

The greater reaction rates in high mass stars not only consumes hydrogen at a greater rate than in low mass stars, it also limits the amount of hydrogen available to the star. Convection can carry stellar material down deeper into the star, and in low mass stars this makes nearly all the hydrogen in the star available for fusion. Material doesn't really move around in the radiative zones, however. Fresh hydrogen from the surface of a star like the sun will never get to the core, reducing the amount of hydrogen available to it over it's life. Once hydrogen in the thermonuclear core has been completely converted into helium, energy production slows and gravity causes the star to contract. This raises the pressure around the core enough to let hydrogen fusion occur in a shell around the core, driving an overall inflation of the star. A Sun-like star will fluff up until it is the equivelant size of Mercury's orbit. The star is now known as a red giant, growing brighter but cooler and leaving the main sequence in a characteristic branch on the HR diagram.

Eventually the core contracts enough for helium tobegin fusing into carbon and oxygen. This is carried out via a reaction that requires higher temperatures and yields much less energy per reaction. However, the rate of this reaction is so high that it more tha makes up for the lower yield. The star is now it it's horizontal branch phase, where it will remain burning helium for about 10 percent of its former life on the main sequence.

Sometime during this period, the star will undergo oscillations similar to those of Cepheid variables, but at much lower luminosities. Is has become an RR Lyrae variable, whose nearly constant luminosity provide helpful "standard candles" for gauging distances within the Milky Way. All too soon, the stars supply of helium is exhausted in the core, and it again contracts, driving helium burning in the shell. All this shell burning again boosts the stars luminosity, and the star enters the so-called asymptotic giant branch. A Sun-like star at this stage would be a distended giant as big as the orbit of Mars.

Instabilities within the star at this point lead to cycles of alternating helium and hydrogen fusion in different layers in the star. The resulting thermal pulses create tremendous winds, blowing stellar material off the surface of the star, exposing the hot core and creating planetary nebula.

For more massive stars, pressures and temperatures are high enough to continue the process of fusing ever heavier elements in their cores, and sustaining multiple shells of fusing element. As this happens, the density in the core decreases, and gravity compresses the star to compensate. This increases the rate of thermonuclear reactions, hastening the demise of the star. A 25 solar mass star, for instance, consumes the hydrogen in its core in about 7 million years. Helium fusion consumes the helium in the core in only 500 thousand years. Oxygen fusion, producing silicon, consumes the oxygen in the core in only 6 months, while the resulting silicon is fused into iron in a single day.

During these various power plays, the massive star's surface temperature swings wildly from being red-cool to blue-hot and back, while it's total luminosity creeps ever upward. At times, it enters periods of instability and it undergoes regular pulsations as a Cepheid variable. All too soon, though, it hits the end of the line. Heavy element fusion in the star is driven by high temperatures and pressures, but releases much less energy. Iron, the result of silicon fusion, obstinantly refuses to fuse into anything greater. The temperatures and pressures required to fuse iron demand energies greater than those that are released by iron fusion, and nuclear fusion stops. The newly dormant iron core, unable to maintain its thermal pressure, implodes in less than a second.

The resulting release of gravitational energy is stupendous. A shock wave ripples through the star, detonating a supernova explosion of titanic proportions. For the next weeks or months, the supernova can outshine the rest of the galaxy in which it resides. Much of this afterglow is the result of elements heavier than iron that were forged in the supernovas intense fires, as nickel and other heavy elements decay into lighter isotopes.

Two-fer plus one! Shapley 1 in real life and Elite

The Fine Ring Nebula, Shapley 1, was discovered in 1936 by Harlow Shapley, and it is the first destination of our tour where real life and Elite Dangerous really diverge. A beautiful planetary nebula blown off from a Sun-like star at the end of it's life, Shapley 1 provides an example of the fate of our own Sun at the end of its life.

The nebula in Elite is visually very similar to real life images, but differs in distance and in the star at its core. Current estimates place Shapley 1 at about 4900 light years from Earth, while the nebula lies a mere 1000 light years away in Elite. Strangely, while the real life Shapley 1 surrounds a white dwarf - the cooling core of the Sun-like star that created it. In Elite, however, the nebula surrounds a Wolf-Rayet star - a massive blue giant nearing the "iron flash" that will result in its supernova. Some Wolf-Rayet stars do produce planetary nebula as they blow off their outer layers, so we can forgive Frontier this.

Extremely hot compared to the Sun (5780 degrees Kelvin), or even the main sequence O type star that bred it (30,000 degrees Kelvin), Wolf-Rayet stars at the center of planetary nebula typically have surface temperatures of 80,000 degrees Kelvin or more. These stars are actively losing material from their surface, and will supernova in relatively short times (though still in the hundreds of thousands of years).

While there are closer Wolf-Rayet stars in Elite, Shapley 1 is particularly beautiful, and well worth the trip.

Omicron Eridani B - the Shapley 1 that should be

While Shapley 1 bears a Wolf-Rayet star in its core in the Elite galaxy, the real life nebula surrounds a white dwarf. In lieu of the Shapley 1 white dwarf, let's take a trip to the first white dwarf to be discovered, Omicron Eridani B.

As Sun-like stars age, they burn helium in their core and then a shell surrounding their core, producing sufficient energy to blow off the outer portions of the star. Known as "envelope ejection", this produces distinctive planetary nebula, and exposes the brilliantly hot (but relatively dim) core of the star. This core gradually cools, but remains distinctively white. On the HR diagram, white dwarfs make up about 10 percent of the stars in the Galaxy. When Hertzsprung began his work, however, white dwarfs were unknown, because they are so dim that none are visible to the naked eye. When white dwarfs were first discovered telescopically, they were originally thought to be type M stars, red dwarfs. While Omicron Eridani B was discovered in 1783 by William Herschel, it wasn't until 1910 that it was discovered by Henry Norris Russel, Edward Pickering, and Williamina Fleming that the star was white, and not red.

As the core of a former sun, Omicron Eridani B is remarkably dense, packing half a solar mass into a sphere with a radius of only 1.4 percent of the Sun. While the star is brilliantly hot (16,500 degrees Kelvin), it is dim, with a total luminosity of only 1.3 percent of the Sun.

Next: We live in a spiral - mapping the arms of the Milky Way.
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This is fantastic, thanks for sharing! I've only read the first 3 parts so far but it's great reading so keep it up!

Vega requires a permit from the Federation, which I don't have, and so I had to be satisfied with a visit to the star Altair.
If I remember correctly, that permit is quite easy to get. You get it at Chief Petty Officer, which is the fifth Federation rank and didn't take too long to get to when I did it. Granted this was a long time ago though and missions have undergone several changes since then, so I don't know what the rank missions are like now.
Part VII - We live in a spiral

Mapping the arms of the Milky Way

Astronomer Edwin Hubble - the same man who used Henrietta Swan Leavitt's Cepheid period-luminosity relationship to determine that there were other galaxies outside our own - was fascinated by galaxies. Using the 100 inch telescope at Mount Wilson, as part of his research into cosmology, he cataloged an almost uncountable number of galaxies. He noted that galaxies came in different types, and developed the most commonly used system for classifying galaxies: The Hubble Sequence.

The Hubble Sequence divides galaxies into three main classes: Elliptical galaxies, which have a smoothly rounded shape; spiral galaxies, which look like a flattened disk with stars forming a (usually two-armed) spiral; and lenticular galaxies, which have no visible spiral structure. There are also a class of galaxies called irregulars, which defy classification.

The idea that the Milky Way was a spiral galaxy had been around a long time, but concerted efforts to prove this had failed. In 1852, American astronomer Stephen Alexander first suggested that our galaxy was also a spiral. A number of astronomers attempted to prove this, largely falling back on Kapteyn-style star counts. Dutch astronomer Bart Bok spent over a decade counting stars in different directions. He hoped and expected that the number of stars he counted would shoot up when he reached a spiral arm and then fall when he passed beyond it. It seemed like a reasonable assumption, because the arms of a spiral galaxy were brighter than the rest of the disk, implying that the arms had more stars. Bok, however, was unable to discern more than a hint of an arm.

It was Walter Baade, studying the Andromeda Galaxy, who provided an alternative. Baade found that the blue O and B supergiants lined only Andromeda's spiral arms. Ordinary stars, which Bok had been counting, may not trace the galaxy's spiral arms at all, but these bright blue stars do. Baade's idea to determine the shape of the galaxy would require determining the distances to the galaxy's bright OB stars, and Baade never pursued this approach himself. But his suggestion caught the attention of William Morgan, an astronomer at Yerkes Observatory in Wisconsin. A skilled observational astronomer, Morgan and his colleagues had developed a careful scheme for classifying stars according to their spectral type and absolute magnitude.

To map the arms as Baade had sugfested, Morgan needed to determine the distances of the high-luminosity O and B stars that lit the arms. Most of these lie too far away to measure paralaxes, so another way had to be developed. Fortunately, stars of different absolute magnitudes have slightly different spectra. For instance, B-type supergiants have narrower hydrogen lines than the less luminous B-type stars. Morgan had earlier embarked on a massive project to refine the spectroscopic classification of stars. By scrutinizing a spectrum, he could determine the star's absolute magnitude and, from that, it's distance from Earth. The modern MK system of classifying stars - calling the Sun a G2V star, for example, or Arctarus a K0III, was developed by Morgan, who is the "M" in "MK".

In 1947, Morgan began to use this system in an attempt to map out the galaxy's spiral arms. From 1947 to 1949, Morgan and Jason Nassau discovered and classified hundreds of O and B stars. In 1950, the two presented their results during a meeting at the University of Michegan. While these did not show more than a hint of a spiral arm, they set the stage for additional attempts which stemmed, again, from Baade's work on the Andromeda Galaxy. At the same 1950 conference, Baade displayed a spectacular photograph that caught Morgan's attention. The photograph showed Andromeda's H II regions - glowing areas where hot O and B stars had ionized the interstellar hydrogen near them. Hydrogen atoms come in two varieties: neutral hydrogen, or H I, consisting of a proton and an electron; and ionized hydrogen, or H II, when the proton has lost its electron. Neutral hydrogen pervades interstellar space, but a single O or B star can ionize all the hydrogen for dozens or hundreds of light years around. The famous Orion nebula, for example, shines because of the ionizing action of the O and B stars it harbors.

Baade's photograph of Andromeda's H II regions showed that they defined the spiral arms, marking them even better than individual OB stars did. Morgan decided that the best way to map the galaxy would be to identify large H II regions of our galaxy, and find the distance to them by the spectroscopic classification of the stars within them. Each of these H II regions has not just one S star but a lot of O stars, providing ample opportunity for Morgan and his colleagues. In 1950 and 1951, Morgan's colleagues Stewart Sharpless and Donald Osterbrock began to photographically map out the H II regions of the galaxy with cameras equipped with filters that transmitted red light. Even though the O and B stars that ionize the H II regions are blue, most H II regions are red. The red filter emphazised the H II regions on the resulting photographs and made them easier to find. After searching the plates for H II regions, they began the careful work of determining the distances to the O and B stars within them.

Morgan's map, which he plotted in 1951, revealed two spiral arms. The Sun lay along the inner edge of one, now called the Orion arm, which stretched from the constellation Cygnus to Monoceros and contained H II regions such as the North America Nebula and the Orion Nebula. A second spiral arm, the Perseus arm, ran parallel to the Orion arm and lay some seven thousand light years further from the galactic center. Morgan also saw hints of a third arm, interior to the Orion arm, that ran through Sagitarius.

Morgan's team had succeeded where Bart Bok and his years of star counting failed, because he determined that stars don't define the spiral arms, H II and OB stars do. Bok didn't realize that. Though O and B stars light the spiral arms and make them bright, ordinary stars are no more common in the arms than between them, so counting ordinary stars can't reveal spiral structure. Later work in the new field of radio astronomy has since fleshed out this early map, because the dust that blocks light from the farther parts of the galaxy doesn't hinder radio waves,

Across the arms

Caveat: These screenshots were taken with my old HD5770 video card. A more powerful video card makes them look much nicer!

Two prominent H II regions within easy access are the North America and Trifid Nebulas.

The North America Nebula is an H II region in the constellation of Cygnus. Along with the nearby Pelican Nebula, it forms one of the closer H II complexes to the sun, sitting about 1800 light years away. The nearby A2 Ia star Deneb provides energy to ionize this region and make it visible.

The Trifid Nebula is an H II region in the constellation of Sagittarius. It is a spectacular combination of an open cluster of stars, an emission nebula (the ionized H II region, red), a reflection nebula (the upper, violet portion which shines by reflected light of the stars nearby), and a dark nebula (the dark 'veins' that cause the trifurcated appearance and give the nebula its name). The Trifid Nebula is a star-forming region on the near edge of the Sagittarius arm of the galaxy, a little more than 5000 light years from the sun.

I chose the North America Nebula because it is one of the nearer H II regions that Morgan used to map the Orion arm. The Trifid Nebula, besides being beautiful, has historical significance for it's use in mapping the Sagittarius arm. It is also the location of the current base camp for the Distant Worlds expedition, being visited by some 700 plus pilots.

Next: The Galactic Arms Race.
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