stellar remnants

anonymous asked:


Firstly, thank you very much! Secondly, a white dwarf is the stellar remnant of a star similarly sized to our Sun. If the mass of the star is >4 times the mass of our Sun, it will usually form a Neutron Star. But ignore that for now. When a star runs out of Hydrogen, it begins to swell up, fusing heavier and heavier elements forming a red giant. When it can no longer fuse the elements in its core, the stellar winds shed the outer layer to form a planetary nebula, which leaves a dense white dwarf behind at the centre. A red dwarf is just a small star that has a “low” temperature, so it appears red (The hottest stars are blue). Hope that helps a little.


Ask Ethan #98: When Will The Stars Go Dark?

“How long would it take for stars to cool down after they have exhausted their nuclear fuel? Will there be any ‘black’ dwarfs? Are there any today?”

While the stars exist in tremendous numbers (some 10^23+ in our observable Universe) and great varieties, every star that ever has shone or will shine will someday run out of fuel and die. When that happens, the inner core of the star contracts down to form a tiny, degenerate but very hot object. But even so, no object with a finite amount of energy can shine forever. At some point, even those stellar remnants will cool down out of the visible portion of the spectrum. But how long will that take, how will that happen, and has the Universe been around long enough (yet) so that such an object exists? Answers here.


White Dwarfs

At the end of its life, a star the Sun’s size will expand into Red Giant. In its core it fuses helium nuclei together to make carbon and oxygen, but a star of this mass lacks the energy to fuse carbon any further than this so the fusion process stops. The red giant eventually pushes away its outer layers to form a planetary nebula, and leaves behind its core

The core of the dead star collapses under its own gravity into a tiny remnant - a White Dwarf. These stars are only a few thousand kilometres in radius, a similar size to Earth, and are about 1000000 times denser than the Sun - between 107 and 1011 kg per cubic meter

These stellar remnants are known to accrete matter since they have exceptionally strong gravitational pulls - almost the entire mass of the Sun condensed into a ball the size of Earth would have very strong gravity - devouring whatever gets too close. Some of these stars are in binary systems with much larger stars, just like Sirius A and B pictured above, where the white dwarf may suck matter straight off the other star! If the white dwarf absorbs enough mass it can collapse into a Neutron Star, and eventually a Black Hole

The Sun will become a white dwarf in about 6000000000 years after burning Earth to ashes in its red giant phase. Pleasant thought

Image Credits: Nasa, ESA


Why does gravity move at the speed of light?

“In theory, we know that the speed of gravity should be the same as the speed of light. But the Sun’s force of gravity out here, by us, is far too weak to measure this effect. In fact, it gets really hard to measure, because if something moves at a constant velocity in a constant gravitational field, there’s no observable affect at all. What we’d want, ideally, is a system that has a massive object moving with a changing velocity through a changing gravitational field. In other words, we want a system that consists of a close pair of orbiting, observable stellar remnants, at least one of which is a neutron star.”

According to General Relativity, the speed of gravity must be equal to the speed of light. Since gravitational radiation is massless, it therefore must propagate at c, or the speed of light in a vacuum. But given that the Earth orbits the Sun, if it were attracted to the Sun’s position some 8 minutes ago instead of its present position, the planetary orbits would disagree with what we observe! What, then, is the resolution to this? It turns out that in relativity itself, what we experience as gravitation is also dependent on both speed and changes in the gravitational field, both of which play a role. From observations of binary pulsars, a gravitationally lensed quasar and, most recently, direct gravitational waves themselves, we can constrain the speed of gravity to be very close to the speed of light, with remarkable precision.