The majority of the stars we see in the sky, like Pollux, are outside of the galactic plane. That means that all those stars we see are not in the galactic disk, and therefore are in the galactic halo, whereas Capella, for example, is where?
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asked Nov 11, 2016 at 23:50
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2$\begingroup$ It seems like you think the galactic "plane" is really really thin. (Like a "line".) This is simply not the case: it is HUGE. $\endgroup$– FattieNov 12, 2016 at 2:30
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1$\begingroup$ Thus for example: "Pollux [is] outside of the galactic plane" that would seem to be totally wrong. $\endgroup$– FattieNov 12, 2016 at 2:32
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3 Answers
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The galactic disk, as Riley Jacob wrote, has a definite thickness. It's actually composed of a thin disk $\sim0.3\text{ kpc}$ thick and a thick disk $\sim1\text{ kpc}$ thick, at least (McMillan (2011) has models with data from the Sloan Digital Sky Survey). There's also a central bulge that is even thicker, as the following diagram (from here) shows:
Pollux is $\sim10\text{ pc}$ away from the Solar System, which is about 1% of the thickness of the thick disk (Capella is barely farther). Essentially, it's in the same plane as the Solar System; that's an insignificant distance.
There are stars in the thin disk, thick disk, and halo, which compose different populaitons based on metallicity. In the thin disk are Population I stars, which are high in metals. In the halo (and thin disk, to some extent) are Population II stars, which are lower in metals and on average older than Population I stars. The Sun is a Population I star in the disk. There may also be a sub-population of Population II stars in the thick disk.
Halo stars certainly do not compose the majority of stars in the Milky Way galaxy. Most stars are contained within the thin disk, thick disk, and bulge.
answered Nov 12, 2016 at 0:30
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$\begingroup$ It has recently been suggested that the Milky Way lacks a classical bulge, and instead may possess a pseudobulge formed by the galactic bar. See Shen et al. (2010). $\endgroup$– user14781Nov 12, 2016 at 16:25
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$\begingroup$ "In a galaxy, there is not a central point around which everything orbits" That's the same in a solar system, and in a planetary system. The comparison is that you have (roughly) disks of things spinning around a big blob in the middle; in the case of galaxies (and stars), the matter spinning around them occasionally coalesces to form stars (and planets). I think there's quite a clear comparison there, although it is artistic/poetic rather than strictly correct. You're a killjoy! $\endgroup$ Nov 12, 2016 at 17:40
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$\begingroup$ Sure, OK, but population I stars are not necessarily "newly born" either. The oldest are about 10 billion years old. $\endgroup$– ProfRobNov 13, 2016 at 7:39
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$\begingroup$ Just an fyi: The galactic plane is very roughly defined (ideally there would be equal numbers of stars above and below). By one estimation, the sun itself is about 70 light years above the plane. $\endgroup$ Nov 19, 2016 at 16:28
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This is not necessarily true—the galactic disk is not 2-dimensional. In the case of the Milky Way, this disk is approximately $1-3$ $\mathrm{kly}$ in thickness, as per Rix & Bovy (2013). This means that stars which do not appear within the galactic plane very well may still be within the disk. In fact, almost all stars visible from Earth lie within this disk. See this question for some further information on distant visible stars.
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I'll turn this around for you. The brightest star that is classed as a "halo" or metal-poor population II star is HD 140283, with a visual magnitude of 7.2 and not even visible to the naked eye. Even this is only a halo object in the sense of being a high velocity object whose trajectory will take it back into the halo in the future - it is presently only 190 light years distant.
I completely concur with the other answers, but let me explain why we don't see halo stars, despite them making up about 1% of the stellar population.
The disc of the Galaxy is roughly 1000 pc thick, so to see a naked eye star actually in the halo, it would be further away than about 500 pc (the Sun is not far from the mid-plane) and have to have an absolute magnitude brighter than -2.5.
Such luminous objects are rare, but not unknown; they would need to be O-type main sequence stars or supergiants. But these are young, massive stars and recent star formation has only taken place close to the Galactic plane and so that is where such objects are found.
If our eyes were about 3 magnitudes more sensitive then we would be able to see metal-poor K-type giant stars in the halo.
