Question: what are brown and white dwafs

Sarah Casewell answered on 16 Jun 2014:

Brown dwarfs are failed stars. Stars form from the collapse of a gas cloud and normally, lots of gas comes together, and it gets squashed. As it gets squeezed more and more, it gets hot and then sets alight. This happens to all stars and once alight, they can burn hydrogen into helium.
Brown dwarfs are the remnants of this process. They never get enough mass together to set alight, so they get very hot – 2000 degrees, but then just stay together as a large ball of hot gas, that cools with time. They’re about the size of jupiter, so still very big, but as they cool, molecules form in their atmospheres, and they have clouds, and weather like on Earth. They’re in the middle between stars and planets!

White dwarfs are dead stars. They are what used to be the core of a star like our sun. Stars are held up against gravity by burning hydrogen into helium. This releases heat which keeps the star inflated. When it runs out of fuel (hydrogen), it can’t keep itself up against gravity and so it collapses in on itself.
The core is all that is left and they are very dense, and hot – maybe 100 000 degrees! But when they form, they just cool with time.

Most stars become white dwarfs – they are small – about the size of the Earth and have the entire mass of the sun squeezed into it.
Any gas that is left over that doesn’t become the white dwarf, is ejected into space and becomes a nebula. Eventually new stars will form here.