Stellar Lifecycle of A9V
- Thread starter daryen
- Start date
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Malenfant
Guest
You rang? 
First, it might be an idea for the reader to remind themselves of how stellar evolution works - it's not quite as straightforward as main sequence > red giant > white dwarf.
An A9 V wouldn't be massive enough to go supernova (and therefore wouldn't end up as a neutron star). You're looking at a start with about 1.6 or 1.7 solar masses here.
Roughly speaking, it'd start off A8/A9 with a luminosity of about 8.9 Sols and a radius of 1.5 Sols. The Frost Line would be at 7.54 AU (so beyond this you'd get icy objects and normal gas giants). The initial habitable zone is at 2.98 AU, and the 100D limit for the star is at 1.39 AU.
It would get redder over its main sequence, and end its main sequence after 1.65 billion years as an F7 V. At this stage its luminosity is about 14 Sols and radius is 3 Sols. The Frost Line would be at 9.48 AU, the final main sequence habitable zone is at 3.75 AU, and the 100D limit for the star is at 2.80 AU. So keep this evolution in mind if you're looking at vaguely habitable worlds here - the luminosity increases quite a bit over 1.65 Ga.
It then evolves into a Subgiant - it's in this stage for 40 million years. Initially, it actually shrinks a bit and gets hotter and brighter over the first 10 Ma, but for the rest of the subgiant stage it cools and expands and dims. By 1.69 Ga - the end of the subgiant phase - it's a K0 IV with a luminosity of about 10 Sols and a radius of 4.38 Sols. Frost line is now at 8.31 AU (but peaked at the start of the subgiant phase at 10.8 AU).
Next it expands into a Red Giant (RGB), before settling down as a on the Helium Burning Main Sequence as a Horizontal Branch giant. The RGB stage is pretty rapid - it lasts about 60 million years. The star rapidly expands, and in the last 10 million years or so it goes from K7 III to M0 II to M1 II. At the end of it, its luminosity is about 2170 Sols, radius is 136 Sols (0.63 AU), and the frost line is at 118 AU. This probably won't be good for any planets around it - any icy worlds would be melting and they'd probably be losing their atmospheres. It also starts losing mass at this stage via the solar wind and has lost about 0.1 solar masses by the end of the RGB.
After this, Helium burning begins in the core which causes the star to contract and stabilise somewhat - the star is now of the Horizontal Branch. This stage lasts from 1.76 to 1.82 Ga. It shrinks back down to an orange K3/K4 III star, and is pretty stable for this period. However, its planetary system has probably been wrecked by the RGB stage. The star actually dims and then brightens over this period. At the start and end of the Horizontal Branch, the star is a K4 III with luminosity around 95 Sols, and radius around 16 Sols (frost line is 24.7 AU). During the middle of the HB stage, the star is a K3 III with luminosity of 65 Sols, and radius of 12.8 Sols (frost line at 20.4 AU). It's not really losing mass at a significant rate during the HB stage.
At 1.82 Ga, the star begins the final giant stage - the Asymptotic Giant Branch (AGB). This lasts from 1.83 to 1.86 Ga, so it's only about 30 million years long. Over this time, the star swells up and cools dramatically, going from K5 III to M1 II. Luminosity increases to 2500 Sols, and radius goes up to 150 Sols (0.7 AU), with the frost line now at 140 AU. Through this stage it's a Mira-type variable star that is pulsating in size, so the radius is actually changing on a short timescale throughout this period. It's also losing mass again - by the end of the AGB it's down to 1.5 solar masses.
The AGB ends at 1.86 Ga, whereupon the star gives one last big outward pulsation that (relatively gently) blows away most of its outer layers to form a Planetary Nebula (pretty when viewed from afar) and expose its core as a White Dwarf (which probably won't be more than 1 solar mass).
First, it might be an idea for the reader to remind themselves of how stellar evolution works - it's not quite as straightforward as main sequence > red giant > white dwarf.
An A9 V wouldn't be massive enough to go supernova (and therefore wouldn't end up as a neutron star). You're looking at a start with about 1.6 or 1.7 solar masses here.
Roughly speaking, it'd start off A8/A9 with a luminosity of about 8.9 Sols and a radius of 1.5 Sols. The Frost Line would be at 7.54 AU (so beyond this you'd get icy objects and normal gas giants). The initial habitable zone is at 2.98 AU, and the 100D limit for the star is at 1.39 AU.
It would get redder over its main sequence, and end its main sequence after 1.65 billion years as an F7 V. At this stage its luminosity is about 14 Sols and radius is 3 Sols. The Frost Line would be at 9.48 AU, the final main sequence habitable zone is at 3.75 AU, and the 100D limit for the star is at 2.80 AU. So keep this evolution in mind if you're looking at vaguely habitable worlds here - the luminosity increases quite a bit over 1.65 Ga.
It then evolves into a Subgiant - it's in this stage for 40 million years. Initially, it actually shrinks a bit and gets hotter and brighter over the first 10 Ma, but for the rest of the subgiant stage it cools and expands and dims. By 1.69 Ga - the end of the subgiant phase - it's a K0 IV with a luminosity of about 10 Sols and a radius of 4.38 Sols. Frost line is now at 8.31 AU (but peaked at the start of the subgiant phase at 10.8 AU).
Next it expands into a Red Giant (RGB), before settling down as a on the Helium Burning Main Sequence as a Horizontal Branch giant. The RGB stage is pretty rapid - it lasts about 60 million years. The star rapidly expands, and in the last 10 million years or so it goes from K7 III to M0 II to M1 II. At the end of it, its luminosity is about 2170 Sols, radius is 136 Sols (0.63 AU), and the frost line is at 118 AU. This probably won't be good for any planets around it - any icy worlds would be melting and they'd probably be losing their atmospheres. It also starts losing mass at this stage via the solar wind and has lost about 0.1 solar masses by the end of the RGB.
After this, Helium burning begins in the core which causes the star to contract and stabilise somewhat - the star is now of the Horizontal Branch. This stage lasts from 1.76 to 1.82 Ga. It shrinks back down to an orange K3/K4 III star, and is pretty stable for this period. However, its planetary system has probably been wrecked by the RGB stage. The star actually dims and then brightens over this period. At the start and end of the Horizontal Branch, the star is a K4 III with luminosity around 95 Sols, and radius around 16 Sols (frost line is 24.7 AU). During the middle of the HB stage, the star is a K3 III with luminosity of 65 Sols, and radius of 12.8 Sols (frost line at 20.4 AU). It's not really losing mass at a significant rate during the HB stage.
At 1.82 Ga, the star begins the final giant stage - the Asymptotic Giant Branch (AGB). This lasts from 1.83 to 1.86 Ga, so it's only about 30 million years long. Over this time, the star swells up and cools dramatically, going from K5 III to M1 II. Luminosity increases to 2500 Sols, and radius goes up to 150 Sols (0.7 AU), with the frost line now at 140 AU. Through this stage it's a Mira-type variable star that is pulsating in size, so the radius is actually changing on a short timescale throughout this period. It's also losing mass again - by the end of the AGB it's down to 1.5 solar masses.
The AGB ends at 1.86 Ga, whereupon the star gives one last big outward pulsation that (relatively gently) blows away most of its outer layers to form a Planetary Nebula (pretty when viewed from afar) and expose its core as a White Dwarf (which probably won't be more than 1 solar mass).
daryen
SOC-14 1K
Yes. Thanks.Originally posted by Malenfant:
You rang?
I am assuming that "Ga" translates into "billion years". Is that right?
Assuming that, I just have a simple follow-on question: As the star transitions from an A9 to an F7, does it go from A9 -> F0 -> F1 -> F2 etc.? Or is there a different progression?
Thanks.
Oh, I can't resist.
M
Malenfant
Guest
Yeah:
Ga = Giga-annum = 1 billion years.
Ma = Mega-annum = 1 million years.
They're commonly used in geology and astronomy.
Yes, the star does go linearly from A9 to F7 V. It only gets strange in the giant stages.
Complex life is rather unlikely. The habitable zone is moving out quite quickly, if the planet was on the outer edge of the initial hab zone then it would last longest because it might just end up being at the inner edge of the hab zone at the end of the main sequence, but it would have had a fair bit of climatological change. Hardy single-celled lifeforms might survive, but it's unlikely that you'd get anything better. It'd probably be like Earth a few billion years ago - nothing on land, everything in the seas (if they survive), and either an aerobic or anaerobic atmosphere.
Ga = Giga-annum = 1 billion years.
Ma = Mega-annum = 1 million years.
They're commonly used in geology and astronomy.
Yes, the star does go linearly from A9 to F7 V. It only gets strange in the giant stages.
Complex life is rather unlikely. The habitable zone is moving out quite quickly, if the planet was on the outer edge of the initial hab zone then it would last longest because it might just end up being at the inner edge of the hab zone at the end of the main sequence, but it would have had a fair bit of climatological change. Hardy single-celled lifeforms might survive, but it's unlikely that you'd get anything better. It'd probably be like Earth a few billion years ago - nothing on land, everything in the seas (if they survive), and either an aerobic or anaerobic atmosphere.
