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One thing to keep in mind is that objects that are bound gravitationally actually revolve around each other around a point called a barycenter. The fact that the earth looks like its revolving around the sun is because the sun is much more massive and its radius is large enough that it encompasses the barycenter. This is a similar situation with the Earth ...

31

Anything the mass of a star is going to get hot like a star and fuse hydrogen like a star. In other words it will be a star not a planet! While it's technically possible to have a rocky planet the mass of a star, in practice when stellar systems form there aren't enough metals available to build such a large object. Large objects are invariably built from ...

26

The answer kind of depends on how old you are. At a very introductory level, say, maybe middle school or younger, it's "okay" to refer to Jupiter as a failed star to get the idea across that a gas giant planet is sort of similar to a star in composition. But around middle school and above (where "middle school" refers to around 6-8 grade, or age ~12-14), I ...

22

Not quite like in the photo above, which shows more than what the naked eye can see, but yes, absolutely! Our galaxy (well, the chunk of it visible from these parts) is a naked-eye object. The fact that your question even exists shows how much time is now spent by people under light-polluted skies. It will not be visible from the city, however. You need to ...

15

The estimates I've read are similar to yours: 200 to 400 billion stars. Counting the stars in the galaxy is inherently difficult because, well, we can't see all of them. We don't really count the stars, though. That would take ages: instead we measure the orbit of the stars we can see. By doing this, we find the angular velocity of the stars and can ...

15

When astronomers started to get spectra of stars and began classifying them, the initial classification was based on the strength of the Balmer absorption lines in the spectra. The Balmer lines are created by electons in hydrogen atoms that are currently in the second energy level (N=2) absorbing energy and jumping up to higher levels. The stars with the ...

12

Almost all exoplanets observed are near F, G, and K stars. In part, this is because astronomers are looking for earth-like planets, so they look at stars similar to our Sun, but there are also some physical reasons. Sahu et al (2006) have provided some evidence that red dwarfs (class M) are more likely to have planets than other spectral types, though it is ...

12

The analogy is facile. Helium fuses at a temperature ($10^8\ \text{K}$) roughly ten times higher than hydrogen ($10^7\ \text{K}$), so a better analogy would be alcohol and thermite. That higher temperature is achieved only by massive gravitational contraction after hydrogen fusion [EDIT: in the core] is exhausted. EDIT: To expand, different mass stars ...

10

Given what we know about planetary formation (Link 1, Link 2, Link 3 and Link 4), and the theories around it, it would probably be a safe bet to say that ALL stars end up having some left over material that might become planets. I think the bigger question is how many of those planetary orbits stay stable enough throughout the life of the star? All these ...

10

For single stars (doubles also exhibit the spin-to-orbital angular momentum transfer), Rotation braking states: Stars slowly lose mass by the emission of a stellar wind from the photosphere. The star's magnetic field exerts a torque on the ejected matter, resulting in a steady transfer of angular momentum away from the star. A first order approximation ...

10

If they're sitting still, and are very bright, they are planets. Install Stellarium on a computer or a smartphone. First time you run it on a computer, enter your location in the settings (no need to do that again after the first time); on the smartphone, it deduces the location automatically each time. The program will show you what planets are visible at ...

9

The orbits would be elliptical. What kind of elliptical depends on the distance of the stars from each other and their velocities at a given point. At any given time, if you draw a straight line between the two stars, that line will pass through the barycenter of the system (the center of mass). The stars will move more slowly when they are farther from the ...

9

The IAU is responsible for naming conventions and certifying proposed names. There are certain themes for different classes of objects- all new features on Venus must be named after women, the moons of Uranus are named after Shakespeare characters, kuiper belt objects are named after deities, etc. It is usually the discoverer who chooses the name to be ...

9

Because the stars each form from their own region of the cloud. Each star forming event is separate* from the other stars that are not in the same orbital plane. All the stars are not in a disk like the planets are. This PDF (OBSERVATIONS AND THEORY OF STAR CLUSTER FORMATION) may give you the answers you are looking for. *notwithstanding multiple star ...

9

Y-dwarfs are a subtype of brown dwarfs, which don't produce energy like (or certainly don't produce as much as) normal stars. Brown dwarfs have an upper limit and a lower limit on their masses. Both these limits are informal and approximate. These limits aren't like planets, for which the IAU has an accepted definition, but they're reasonably well-defined. ...

9

The key as I understand it is metallicity. The Big Bang produced virtually nil above helium, so Pop III stars and their ancestral HI clouds had almost no metals. The forest of emission lines produced by even a tiny fraction of metal atoms acts to increase the cooling efficiency of the cloud enormously. At the extreme rate of cooling of modern clouds, ...

9

I think there are really three questions that need to be answered for this to make sense: is there a "normal" limit to how large a star can be? how can population III stars form with such large masses? how can population III stars retain their large masses? An answer to the first question is tricky. We expect large stars to be rare, and the largest ...

9

Typically, a star (or stellar remnant, such as a neutron star, white/black dwarf, or black hole) will be the most massive thing in the area, by far. Planets, even gas giants, are a small fraction of the mass of a typical main sequence star. Now, as in Hal's answer, the relative mass of the planet and its star does make the center of mass, the barycenter, of ...

8

The bigger dip comes when the cooler star passes in front of the hotter object. The reason the dip is larger in this case is the amount of light given off from the area of the hotter star which is covered by the cooler star is much larger than the amount of light given off by the same area on the cooler star. Thus when the cool star passes in front of the ...

8

In the first case, in regards to star measurement, I believe you're thinking of how the diameter of very large stars are measured using interferometry. Because light waves from the edges of these stars arrive at us in parallel, and they are waves, we can determine the diameter of the star by measuring the interference pattern between these light waves. ...

8

The only visible satellites by the naked eye are very low in orbit. That means they are going to move very rapidly across the sky, usually in no more than 10-15 minutes, sometimes even quicker. Also, in order for a satellite to be visible, it has to be the right time of day. It must be night on the ground, but day above the ground. This typically happens ...

8

You've asked a lot of questions there, and I'll try to answer them one by one. First, though, I want to ask what post you're reading about metallicity in the core vs. out here in the 'burbs because I don't think it is correct. Obviously, for example, we exist and we're ~26,000 light-years (half-way out) from the galactic center and we have a fair amount of ...

8

The nuclear fusion that powers stars has little to nothing to do with electrons. In the cores of stars, temperatures are high enough that all the electrons are stripped from the nuclei, leaving a pure plasma. As stars contract and condense out of interstellar dust, their gravitational potential energy is converted to heat faster than this heat can be ...

7

If you think about it logically, it should be easy to visualize. In fact, the brighter star does not have to be larger necessarily. It could very well be smaller- perhaps the larger star is a red giant, while the smaller star is a blue main sequence, which has higher luminosity. In any case, the middle point of the M occurs when the star with a lower ...

7

Yes all the stars you can see belong to the Milky Way galaxy. The Sun is located about two-thirds of the way out from the galactic center. Thus the band across the sky known as the "Milky Way" (from whence the galaxy name came) is the main disk of the galaxy. However, all the individual stars we see are still inside that disk, we just happen to be closer ...

7

not currently. The only images we have are of extended red and blue giants, and then only just via interferometers and the largest telescopes. In maybe 10 years space born infra red interferometers might be imaging these objects but probably more like 20 to 30 years... The best resoution we have managed so far in the optical is around 0.5 milli ...

6

I did some hunting and followed the paper trail to Cook et al. (1995). In Section 4, they identify a class of stars that brighten aperiodically. They reckon that these "blue bumpers" are Be stars: B-type stars that show strong emission lines. Then again, this is off one paper and I'm really not sure if this is a widely accepted view, but I imagine there ...

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