Hot answers tagged

51

Yes, the shape of the constellation does and will change over time. All the stars have their own peculiar velocities and have some random motion which over time will ruin all the constellations. However, Even though the stars are moving at rapid speeds, to us, in our sky, due to their enormous distance from us, they appear to move extremely slowly and the ...


42

In stellar astrophysics, "burning" means nuclear fusion, not chemical combustion. So a star burns hydrogen to helium. (Incidentally, normal chemical burning of hydrogen in air produces water). This might seem to be confusing terminology, but it's not an issue in practice because the regions in stars where fusion takes place (the core, and shells ...


20

Here's an animation I found that gives you an idea of the movements and timescales involved: It depicts the estimated movement of the Orion Constellation from 3 million years in the past to 3 million years in the future.


14

To be fair, Sheldrake credits Greg Matloff (2015) for this "dark matter is really the motions of 'volitional stars'" idea. It's easy enough to show this won't work (I mean, aside from all the nonsensical physics involved), because dark matter is not just "stars in the outer parts of galaxies are moving funny" -- it's "all things in ...


14

Firstly, thank you for your leveled and clear explanation of Sheldrake's essay. I agree with you that it is quite ridiculous to make such a bold claim when there is such little support for it even for small examples, but that's nothing new to humans.... ;) Now, your question, is there any astronomical evidence that they don't? It must be made very clear, ...


8

A black hole of a given mass will probably have arisen from the collapse/supernova of a much more massive star. In particular, stars with an initial mass of less than around 15-20 solar masses are unlikely to leave a black hole remnant at all. Stars of $<8$ solar masses end their lives as white dwarfs and those with $8$ to $\sim 15$ solar masses likely ...


7

Yes, it's the same thing. Usually the phases a star goes through (if big enough) are the hydrogen burning helium burning ... and the phase is named after the element which is being fused into a heavier element. Thus hydrogen burning is the phase where hydrogen is fused into helium. Helium burning is the phase where three helium are fused into carbon; ...


7

The pages you’re looking at describe fusion of two similar nuclei with each other (e.g. oxygen with oxygen). But fusion doesn’t have to involve identical nuclei, and reactions involving more abundant nuclei will in general occur at a higher rate (though this can vary based on other factors, too). Long before oxygen-oxygen or neon-neon reactions become ...


7

There are far fewer than 0.2% of stars that are as massive as $8M_\odot$. It is possible you've confused it with the percentage of stars born with $M>8M_\odot$. The thing is, those massive stars die very quickly, but most stars, those with masses $<1M_\odot$, are still alive. So the fraction of massive stars in existence now is much lower. i.e. Almost ...


6

Other answers have provided general ideas about how to confirm that individual sources are distant galaxies and not clusters, so I'll focus on the question of how astronomers in the Dark Energy Survey (DES) might be confident about galaxy identifications for the hundreds of millions of sources in their catalogs -- particularly as the overwhelming majority of ...


6

This is a great series of questions! Such a low mass black hole (BH) could have originated from a few possibilites: 1) a result of stellar evolution (the resulting black hole mass depends fundamentally on the initial mass and metallicity of the stellar progenitor, among other things); 2) a star collapsed into a neutron star which can accrete matter from its ...


4

The Milky Way takes 225-250 Million years for one rotation (at Sol's radius). Humanity has been here, and looking up, for, at most (and to make the numbers easier) 10,000 years. 10,000 / 250,000,000 = 0.00004 We (humanity) have been seeing stars for 4 hundred-thousandths of a revolution, or 0.0144 degrees of rotation. The constellations change, we just haven'...


4

If you consider a radially symmetric mass distribution, the gravity experienced at a distance $R$ from the center is caused by the mass inside the sphere with radius $R$, thus the mass $$M(R) = \int_0^R \varrho(r) 4 \pi r^2 dr$$ In a radial-symmetric case one can show that the contribution of all masses outside $R$ cancel each other. Thus the experienced ...


4

Actually, it doesn't. If a black hole and a star have the same mass, their gravity is the same. As the formula for the attraction (gravity) between two objects is given as $F=\dfrac{GMm}{r^2}$, where $G$ is the gravitational constant, $M$ is the mass of the black hole/star, $m$ is the mass of the smaller object, and $r$ is the distance between the two. Even ...


4

First thing, Proxima Centauri cannot go supernova. It is only $0.12 M_\odot$, while core-collapse supernovae can only be triggered by stars that are more massive than $8 M_\odot$. Now the only exception here is if it undergoes a type Ia supernova, but this is very unrealistic because Proxima won't become a white dwarf in trillions of years and is not part of ...


4

A spaghettified object may escape; indeed, this is fairly common. The reason is that most objects approaching a black hole do it freely falling along hyperbolic orbits that swing by and then go back out to infinity (unless the closest distance is just a few Schwarzschild-radii out - then it may plunge in). When it passes the tidal radius it begins to be ...


3

This site shows the colours that stars would have if their intrinsic spectra were viewed. The simulations do not have any intervening atmosphere and assume that the star is bright enough that the physiological effects that mean colour vision doesn't work at low light levels can be ignored. What stars look like through the atmospheres of their own planets ...


3

We measure the distance. A cluster is a group of stars in close proximity to each other. For relatively near stars you can directly measure the distance via the parallax. That's what the Gaia observatory has been doing with unprecedented accuracy during the last years. Thus anything which we can measure a parallax for is known - anything we don't measure a ...


3

Although I will only tackle one part of the question, I find the following part of a picture from NRAO/AUI/NSF, S. Dagnello, cited from space.com worth sharing: You see the radial structure of Antares, a red supergiant of spectral type M1.5Iab-Ib, and more specifically The average temperatures of photosphere, chromosphere, and above are given. One can see ...


3

Just for fun I'll plot the data cited in @planetmaker's answer and use the equation there to plot the gravity. We'll apply Newton's shell theorem to their equation: $$g(R) = \frac{G}{R^2}\int_0^R \varrho(r) 4\pi r^2 dr = \frac{Gm(R) }{R^2} $$ where $m(R)$ is the mass enclosed by a sphere of radius $R$. I plotted using this script. It prints out 31.1 ...


2

If you don't consider how the black hole has formed, then it is quite possible for a black hole to form a bound state with a massive object like a star. If the star has a much higher mass than the star, both can circle (or ellipse) around their center of mass. If this COM lies close to the star then the BH will be orbiting around the star. There will be no ...


2

Yes. Insofar as such thing might be possible (perhaps in the style of the novel “The Mote in God’s Eye”), and assuming a star that has a spherically symmetrical mass distribution: Gravity inside a star acts like if all the stuff in the shell of matter farther from the star’s center than you wasn’t there at all. You feel attraction only from the stuff “...


1

Wolf-Rayet stars have long been a subject of controversy in astronomy. Observationally, they are the class of stars that astronomers identify as being very luminous, having very little hydrogen, and are mostly composed of helium, carbon, nitrogen, and oxygen. This class of stars remained an enigma until the 1980s, when Wolf-Rayet stars started to be ...


Only top voted, non community-wiki answers of a minimum length are eligible