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Why do we know #Betelgeuse may go #Supernova within the next few 100000 years, but most likely not soon? And why are we sure the Sun will be a safe star for Billions of years?

A brief primer #thread to Stellar Evolution. 1/

Sun: SDO. Betelgeuse: ESO/ALMA
#astronomy #Space
First thing to know is: stars are not all alike. They are big, small, bright, dim, have different colors and die different deaths. The quantity at the root of their diversity is their MASS. Stars have masses between roughly 0.1 & 100 times the Sun’s mass. Large ones are rare.2/
Here is the fundamental diagram of stellar evolution. It’s called the Hertzsprung-Russell Diagram (HRD). The further left you go, the hotter stars become. The further up you go, the brighter stars become. 3/
(Pic: ESO)
You will notice the diagonal band in the HRD. It’s called the “Main Sequence”. This is where stars spend most of their lives. On the Main Sequence, they convert Hydrogen to Helium in their cores through nuclear fusion. 4/
Here’s where mass comes into play: massive stars (towards top left in the HRD) are more efficient at converting H to He. They hence are disproportionately brighter than lighter stars. Think that massive stars meet conditions for fusion over a larger fraction of their body. 5/
The relation goes: a star X times more massive than the Sun is X to the 3.5th power brighter. If #Betelgeuse had roughly 20 solar masses while on the Main Sequence, it would have been ~36 000 times (!) brighter than the Sun. 6/
Of course, this means that massive stars go through their fissionable Hydrogen way quicker: a star X times more massive than the Sun is X runs out of Hydrogen X to the 2.5th power faster. 7/
The Sun would stay on the Main Sequence for ~ 12 Billion years; a 20 solar mass Betelgeuse only for ~7 MILLION years. Actual numbers differ a little because of other parameters like rotation. Betelgeuse is thought to be ~8 Million years old. 8/
On the Main Sequence, massive, bright stars are also hotter: the biggest ones have photosphere temperatures up to 50 000 K, the Sun ~6000K, a 0.1 solar mass star less than 3000K. 9/
A body’s temperature largely determines its color: hot is blue, cooler is red. This explains why stars on the Main Sequence have different colors. 10/
Once the Hydrogen is exhausted, stars enter the late and final stages of their evolution. In many, different fusion processes occur, they grow to large radii, become cooler, hence red; and brighter: Red Giants or Supergiants, top right in the HRD. 11/
(Pic: @ESO )
@ESO Stars have different ends depending on their masses. Very massive ones can go through advanced stages of fusion, generating heavier elements up to Iron in the process. Inevitably though, eventually they will all run out of fuel. 12/
@ESO A star like our sun will eventually start pulsating and expelling shells of gas that will form a planetary nebula (example: @NASA/@ESA #Hubble image), while the remaining star will become a White Dwarf star that will slowly cool. 13/
@ESO @NASA @esa Heavier stars like #Betelgeuse will eventually suffer a core collapse and explode as a #Supernova, leaving a gaseous shell (credits in image) and a stellar remnant - a Neutron Star for Betelgeuse, for other stars a Black Hole. 14/
@ESO @NASA @esa We know from its position in the HRD that #Betelgeuse is in the Red Supergiant stage of its life. This phase is a lot shorter than the time a star spends on the Main Sequence. For #Beteigeuse, it could be a short as approx. 100000 years. 15/
@ESO @NASA @esa However observations indicate #Betelgeuse's position within its Supergiant life - & none of them suggest that it is currently on the verge of #Supernova, but that a Supernova won’t occur for another ~100000 up to a million years, depending on some hard to measure properties. 16/
@ESO @NASA @esa It is common for stars towards the end of their lives to become unstable and brighten and fade in the process - and most serious scientists think #Betelgeuse’s current dimming is just part of these up-and-downs. 17/
@ESO @NASA @esa Summary: Stellar Evolution is diverse and complex, #astrophysicists have accumulated a tremendous amount of knowledge on the topic. Stars have wildly different lives depending on their mass: 18/
(Pic: @ESO)
@ESO @NASA @esa The few massive stars live short, intense lives with often explosive ends. The common, less massive stars live longer, less brightly & often go out quietly. #Betelgeuse is part of the first group - but most likely won’t end its life for many human generations to come. 19/19
@ESO @NASA @esa An addition, thank you to @Todd_Scheve : An illustration of #Betelgeuse's enormous size, attained during its evolution from the Main Sequence towards its Red Supergiant state.
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