# Tag Info

## Hot answers tagged star

28

Zero. Normal stars are not dense enough to produce strange matter. They have regular matter only (neutrons and protons). Strange matter has been hypothesized to form inside neutron stars, but this is highly speculative. Presently, nobody really knows what's in the nucleus of neutrons stars. Some references: Physics and Astrophysics of Strange Quark Matter (...

19

It is not possible to know. The speed of light is the speed of information. The information "the star has exploded" cannot travel faster than the speed of light, so there is no way to know that a star has gone supernova before that information reaches us. Usually the first particles to reach us from a supernova are actually neutrinos, which can ...

17

"Nearly straight up" suggests Vega. At that time and location it is only 5 degrees from the Zenith. It is a notably bright star, much brighter than any which are nearby. Vega is a bright nearby star system. It isn't known to have planets, but is surrounded by a disk of dust and debris surrounding it. It spins rapidly, pulling it into an oblate ...

15

There are three important sources of energy in planetary bodies (or their moons) that may be important in determining their suitability for life. All of these are evident in various bodies in the Solar System. The first is tidal heating. When one object orbits another then there will be gradient in the gravitational force across their finite sizes. If this ...

9

Let me first underline two specific definitions of @Alexandre: We are looking for "matter", that means a finite region in space in thermal equilibrium. And we are looking for "strangeness", in the sense that strange quarks play a role. That means that we are not only interested in some transient, intermediate process, and it means that ...

9

Both free-free and bound-free absorption are strongly dependent on the atomic number $Z$ of nuclei in the gas. For free-free absorption it is simply that the emissivity per unit volume of electrons accelerating in the field of an ion scales as $Z^2$ (there are $Ze$ electrons accelerating near a charge of $+Ze$). Since by Kirchoff's Law, absorptivity is ...

8

Deuterium burns first because its combination with a proton or another deuteron is a pure strong nuclear force interaction, with the minimum of Coulomb repulsion. It occurs at lower temperatures than protium burning because although getting two protons together requires tunneling through a similar Coulomb barrier, a weak interaction is also required to turn ...

5

Stars function by nuclear fusion. There is energy that isn't released by nuclear fusion, nor from the nuclear decay of elements produced by fusion. There is the cosmic microwave background. This is radiation, but it "cold", at about -268 *C. There is a lot of energy, but because it is so "cold" it is difficult to use it to power ...

5

ProfRob makes a good point about tidal heating, but you can have internal heat sufficient for life even without this. The heat I'm referring to is heat trapped in a planet's core from its formation, which slowly and reliably leaks out through geothermal vents. We observe this at the bottoms of Earth's oceans, where thriving colonies of deep sea organisms ...

5

I will use some concepts form statistical mechanics, I hope you are familiar with some of the concepts. Consider a gas of $N$ particles of mass $m$ with Hamiltonian function $$H(\bar{q}, \bar{p}) = \sum_{i=1}^N {|\vec{p}_i|^2 \over 2 m} + U(\bar{q})$$ Here, $\bar{q} = (\vec{q}_1,\vec{q}_2,... \vec{q}_N)$ are the position of the particles and $\bar{p} = (\vec{... 5 As already noted in another answer, there aren't many studies about the long-term evolution of low-mass stars because we're fairly sure they don't evolve off the main sequence within the life of the Universe. But the answer inspired me to (quickly and roughly) compute the main-sequence lifetimes of some (roughly) solar metallicity objects between 0.15 and 0.... 5 Rob's answer already gives a rough estimate. I will try to make it more precise by relaxing some assumptions. How many star have been born in our universe? The cosmic star formation history$\psi(z)$is a function that gives, for every redshift, the density of star formation rate. It is usually expressed in units of$M_\odotyr^{-1}cMpc^{-3}$. The total ... 5 On average the Milky way has one supernovae per 100 hundred (10^2) years. As the Galaxy is ~10 Billion (10^9) years old, then that is ~10^7 supernovae in the Milky way's life. Assuming the average galaxy looks like the Milky way and that there are an estimated ~1 Trillion (10^12) galaxies in the Universe, then that suggests there have been ~(10^12 x 10^9) / ... 3 You can do this via VizieR (main site at CDS, Strasbourg, France linked; worldwide mirrors are available). You can search by catalog name from that form or enter I/345/gaia2 to get the specific catalog table we want. Assuming you have no other constraints other than what you specified in your question, we can enter the following into the constraint fields: ... 3 You can also have planets with powerful magnetic fields, a conductive body passing through them generates electric currents, that besides simply heating can also power chemical reactions directly (like charging up a chemical battery). There is also chemical energy. If there is a constant supply or accumulated store of reactive chemicals, that can act as fuel ... 3 When looking up the issue, I found surprisingly few publication that deal with such low mass stars and provide figures for their lifespan. Namely, I have found only two: The End of the Main Sequence: (1997) Laughlin, Bodenheimer, Adams It deals with stars up to$0.25M_\odot$, so only the convective ones. Figure 2 contains a small subfigure with the values of ... 2 It could be misleading since not all hot stars are high mass. White dwarf stars are hot compared to most other types of stars; yet they are, by definition, less massive than the stars from which they evolved. The logical implication is the other way around: high mass stars are hotter than lesser mass stars, and therefore have shorter lifespans than lesser ... 2 The answer by mentioned two internal sources of energy in planets and moons, etc.: one) Tidal heating. In some situations tidal heating hasno significant effect, in ohters it can make a cold moon warm enough for life, in others it could cause a runaway greenhouse effect. Two) Energy produced by the decay of radioactive isotropes. But there is at least one ... 2 ...how typical is the solar corona? The Sun is remarkably "average" (e.g., see https://physics.stackexchange.com/a/262732/59023). So my best guess is that the corona is also remarkably "average." Technically the most abundant stars are red dwarfs and the Sun is a yellow dwarf, but dwarf stars are the most abundant and the sun is part ... 2 A quick calculation tells us that a quasi-star with a radius of 10 billion kilometers (a figure suggested by the Wikipedia page) at 4.25 lyr (Proxima's distance from the Sun) would have an angular diameter of 1.7 arcmin, which is about three times Jupiter's. As a comparison, here from left to right there are the apparent sizes of the Sun, the quasi-star and ... 1 A black hole merger can be more powerful. Power is energy divided by time, and energy can come in many forms. So the most powerful events are short, and might not be very hot or bright. A powerful (type II) supernova might emit$10^{46}$joules over 10 seconds. giving a power of$10^{45}\$ watts. Most of this is in the form of neutrinos, with about 1% ...

1

The problem with your argument is that you incorrectly assume that increasing nuclear fusion increases the pairs/photons ratio. It is certainly true that increase in temperature increases the nuclear fusion rates and therefore the production of photons. More photons means greater possibility to produce electron-positron pairs, but why do you think that the ...

1

The heliopause oscillates due to changes in the solar wind (i.e., speed and density) and changes in the local interstellar medium (i.e., speed and density). The next largest influence comes from what are called pick-up ions, which exert an effective pressure on the termination shock etc. These act kind of like cosmic rays do on supernova, i.e., they modify ...

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