A question about bright things

[Jepjep]

Do all celestial bodies have hydrogen and helium? Is that what makes them bright like stars? ex. planets?

[michael.h.f.wilkinson]

Hydrogen and helium are the primordial elements, and all stars and gaseous planets are mainly made up of those two elements. All other elements (apart from a tiny amount of lithium) were made in stars. What makes stars shine are nuclear reactions taking place in the core. In their main sequence phase they are turning hydrogen into helium, during red giant phase heavier elements up to and including iron are formed. All elements beyond iron are formed in supernovae.

The fact that all atoms in our bodies except hydrogen are formed in stars means we are quite literally stardust. It is quite curious that some complex specks of stardust have woken up, wondered about the stars and realised that they are in fact stardust

[Jepjep]

Hydrogen and helium are the primordial elements, and all stars and gaseous planets are mainly made up of those two elements. All other elements (apart from a tiny amount of lithium) were made in stars. What makes stars shine are nuclear reactions taking place in the core. In their main sequence phase they are turning hydrogen into helium, during red giant phase heavier elements up to and including iron are formed. All elements beyond iron are formed in supernovae.

The fact that all atoms in our bodies except hydrogen are formed in stars means we are quite literally stardust. It is quite curious that some complex specks of stardust have woken up, wondered about the stars and realised that they are in fact stardust

What about planets? Why do they glow like stars?

[Owmuchonomy]

Planets reflect light so we see the light from stars like our Sun reflected from their surface. The amount of light they reflect compared to that which they absorb is called the Albedo. For example, Venus has thick cloud cover which is a good reflector and so it appears very bright. It has a high Albedo.

[Jepjep]

That explains it. Thx

[acey]

In our solar system the outer planets (Jupiter, Saturn, Uranus, Neptune) are gas giants consisting mainly of hydrogen and helium, like stars. But they are much smaller than stars, so the pressure at their cores is not enough to start nuclear fusion which would make them emit their own light. The inner planets (Mercury, Venus, Earth, Mars) are rocky because solar heat burned away primordial hydrogen and helium leaving the trace heavier elements from the original gas/dust cloud that formed the solar system. Earth's hydrogen is mostly in water, which may have been brought by comets from the outer solar system in the early period when those objects were much more abundant and would have hit Earth frequently. Helium is nonreactive so Earth retains no primordial helium: the stuff in balloons is a decay product of radioactive uranium, extracted from underground gas pockets.

Carl Sagan popularised the "we are stardust" line (quoting Joni Mitchell). I like Martin Rees's version: "we are nuclear waste" (since the heavier elements were formed by fusion in stars). But since humans are about 60% water by weight, and the hydrogen in that water was made within the first three minutes after the Big Bang, we could say that we're largely Big Bang stuff.

[michael.h.f.wilkinson]

We may be over 60% water by weight, but the weight of the water is mainly oxygen (8/9 or 88.88888889%). That swings the balance in favour of nuclear waste again

[ollypenrice]

The notion of 'waste' is predicated upon the idea that there was a primary intention to do something else, which would leave waste behind. I doubt that anyone contributing to this thread so far really believes that...

lly

[nameunknown]

Ah yes - 95% dark matter and dark energy. 5% left which is mostly H and He and a trivial bit of other stuff. Of course, the "other stuff" makes a lot of noise about how important it is.....