# Tag Info

90

Yes. It does not rotate uniformly though, different portions have a different angular velocity (as a body made of plasma, it can get away with this). Measuring this in theory is pretty easy, we just need to track the motion of the sunspots. This isn't as simple as calculating the changes in relative positions of the sunspots, though, as the Earth is ...

46

The chemical enrichment of the Universe over time is indeed a thing. The plot below (source) shows observational measurements of the cosmic density of ionised Carbon in the Universe against redshift (higher redshift -> further back in time). The abundance of other heavy elements over time shows a similar trend. Stars are explicitly classified based on ...

38

The answer to your question is both yes and no, depending on the circumstances. Two white dwarfs colliding would likely yield a Type Ia supernova, assuming the combined mass exceeded the Chandrasekhar limit ($\sim1.4$ solar masses). The unstable object resulting from the collision could not be supported by electron degeneracy pressure; when the temperature ...

36

Yes, the Sun rotates. This can be observed by tracking a variety of features on the Sun, such as sunspots, X-ray brightpoints, coronal holes, filaments, and small magnetic flux elements. Another way to determine the rotational speed of the Sun is to measure spectral lines at the edge of the Sun's disk and determine their redshift. It is thought that the ...

35

Stars don't "come from" a supernova. Stars come from the interstellar gas in the galaxy, particularly where it is more concentrated into nebulae. This gas is mostly hydrogen and helium, but it is "enriched" with heavier elements from old stars, including from stars that have exploded in supernovae. Over the billions of years since the ...

27

Yes, stars can form outside galaxies if the conditions are right. An impressive example is D100, a galaxy that is moving through a cluster so fast that the ram pressure from the ambient gas forces galactic gas out of it leaving a long tail. That tail is dense and cold enough to allow star formation, and there are newly formed clusters in it. In principle ...

24

The solar system contains very little of elements heavier than Helium - less than 2% by mass. This is reflected in the chemical abundances measured in the photosphere of the Sun. i.e. The Sun does contain heavier elements. Your question is the wrong way around; it is not that the heavier elements have not sunk into the middle, it is that the vast majority ...

19

The mean mass of a star in a typical star forming region is about 0.3 solar masses and contains about 1% by mass of elements heavier than helium A typical core-collapse supernova progenitor might have a mass of around 15 solar masses and they will be responsible for dispersing a few (say 3) solar masses of heavy elements into the interstellar medium. i.e. ...

15

Although its too late to reply to this interesting question but trying to add few more points. Yes the sun rotates. Now the question arises as to how we can check that? We can observe this by observing sunspots. All sunspots move across the face of the Sun. This motion is part of the general rotation of the Sun on its axis. Observations also indicate that ...

15

Cosmic GDP has already crashed, as Peak Star was ~11 billion years ago. According to Sobral et al's prediction, the future star production by mass will give only 5% of the stars in the universe today, "even if we wait forever." More theoretical predictions, such as this one, suggest that nebulae will run out of hydrogen on the order of $10^{13}$ years, ...

15

The structure we see in the Universe has formed from the gravitational collapse of the matter that was once an almost smooth density field of gas ("baryons") and dark matter$^1$. The word "almost" is important here, for if it had been completely — or even non-completely but much more — smooth, then the collapse would not have had the time ...

13

It's more do do with having a higher pressure gradient than a higher pressure, though for a cloud of a set size, the two are equivalent. For a cloud to be in equilibrium requires $$\frac{dP}{dr} = - G\frac{M\rho}{R^2},$$ where $dP/dr$ is the pressure gradient and $M$ and $R$ are the mass and radius of the cloud and $\rho$ is its density. It we just make a ...

12

There are two main theories for the formation of binary stars - one accepted, and one mainly deprecated. The fission hypothesis. The fission hypothesis states that the binary system forms after the collapse of the original gas cloud into a protostar. Angular momentum is conserved, so as the extremely large cloud slowly contracts, it spins faster. After ...

10

Most stars form in clusters, so it is very likely that the Sun was part of a star cluster when it formed. But in On the Dynamics of Open Clusters, the relaxation time of a cluster is calculated to be in the order of $\tau=4\times10^7 \textrm{yr}$. During that time, about one hundredth of the stars will escape from the cluster (i.e. reach escape velocity). ...

10

It's quite possible for stars to form outside of galaxies, typically in environments where large amounts of gas have been stripped from a galaxy. This usually involves either a tidal interaction with another galaxy or the intracluster medium (ICM). In the latter group are a set of peculiar galaxies sometimes dubbed "jellyfish galaxies". Gas, dust and stars ...

9

Yes, unless you want to get really particular with the "protoplanetary" part. For example, there are stars forming in the circumstellar disks around Wolf-Rayet stars [reference]. If we were picky, we might not call the circumstellar disk around this Wolf-Rayet star a protoplanetary disk (and instead refer to the planet-forming circumstellar disks of the ...

9

The Big-Bang was not an explosion in empty space. Inter-galactic space is not empty, there is an inter-galactic medium, gas clouds and material ejected from galaxies, including stars and possibly globular clusters, by various mechanisms...

9

There are two phases to this problem. In order to accrete into stars, a huge amount of angular momentum must be lost to allow so much mass to gather into a small volume. A second problem is how stars like the Sun end up rotating so slowly, when younger versions of stars similar to the Sun rotate much faster. The solution to the first problem may be solved by ...

9

One can make a theoretical upper bound by considering the most short-lived star possible $\tau_{short}$, and a large supply of initial hydrogen $M_H$. Then one could calculate the fraction hydrogen that is recycled $r$ after the star ends (with a supernova), and get a total number of generations as $\log (M_{star}/M_H)/\log(r)$. If one uses the solar-mass \$...

8

No, not really. Stars can form in circumstellar disks, that are, in general, disks surrounding forming stars, but not in protoplanetary disks. Protoplanetary disks are, by definition, flat, rotationing disks composed of gas and dust, found around newly born low-mass stars (see the review by Williams & Cieza (2011)). There are two important point in this ...

8

'Radial direction' typically means from the center, moving outwards. Typically, these kinds of studies will break a galaxy up into several annuli, or rings, and examine the star formation activity within each ring. (Imagine drawing a series of concentric circles, each one a little bigger than the last.) Then, if the star formation rate or star formation ...

8

Regarding the title: Yes. Does this mean that the star started off as a planet? Yes, a star could technically start out as a planet, if it accreted enough mass. However, this is extremely unlikely, since the planet would need to be 80x the mass of Jupiter for it to undergo nucleosynthesis. Stars require hydrogen fusion and earth has little H. Could ...

8

Do stars tend to leave "these stellar nurseries" after a while, and it's only the short lifetime of the most massive stars that keeps them from leaving before going supernova? Yes. The lifetimes of very massive stars (10-50 Myr) are comparable with the dispersion timescale for young clusters and associations. So massive stars tend to die near where they ...

8

It is actually the other way around: First a massive accretion disc can form, through which material looses angular momentum and accretes onto the star radially, hence being angular momentum poor. However, during the initial free-fall phase, before the disc forms, infalling material can be 'rejected' at the star, either via high pressure gradients or ...

7

Yes, Sun has differential rotation. Movement of Sun spots is one of the proofs that Sun rotates. The differential rotation causes the weird twisted magnetic fields which shows in the Sun's prominence.

7

The LMC has an apparent size of about 645x550 arc mins, the SMC 320x205. Both contain several hundred million stars each. The LMC is about 14000ly in size, and is about 10 billion solar masses; the SMC is 7000ly in size, and is about 7 billion solar masses. The visual magnitude of the LMC is +0.28, the SMC is +2.23. Both feature a number of interesting ...

7

"EW" stands for Equivalent Width. The equivalent width of a spectral line is essentially the range of continuum one would integrate over to get the same flux as the spectral line. In your case, this is the Hα line.

7

It is possible for two brown dwarfs to form a contact binary. This would not, of itself, cause core temperatures in either to rise. If the two brown dwarfs were to merge, the mass of the resulting body could be enough to generate sufficient heat and pressure in the core for nuclear reactions to start. If both brown dwarfs had a mass of about 60 Jupiters, ...

7

A 1 solar mass, Earth sized black dwarf would have a surface gravity of about 360 000 g which probably rules out manned exploration by anything we would normally think of as human. For similar reasons, mining would be quite challenging. Another obstacle is that the Universe is not old enough to have produced any black dwarves yet. The oldest white dwarves ...

7

In principle, yes. In practice, no. The question has been studied for a long time. The classic treatment is Dyson's papers on "anchor rings" in 1893 (paper I, paper II) but it goes back to the study of "figures of equilibrium" starting with Newton's considerations of the oblateness of the Earth and then continuing with Maclaurin and ...

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