APOD: Goldilocks Zones and Stars (2020 Jan 31)

[APOD Robot]

Goldilocks Zones and Stars

Explanation: The Goldilocks zone is the habitable zone around a star where it's not too hot and not too cold for liquid water to exist on the surface of orbiting planets. This intriguing infographic includes relative sizes of those zones for yellow G stars like the Sun, along with orange K dwarf stars and red M dwarf stars, both cooler and fainter than the Sun. M stars (top) have small, close-in Goldilocks zones. They are also seen to live long (100 billion years or so) and are very abundant, making up about 73 percent of the stars in the Milky Way. Still, they have very active magnetic fields and may produce too much radiation harmful to life, with an estimated X-ray irradiance 400 times the quiet Sun. Sun-like G stars (bottom) have large Goldilocks zones and are relatively calm, with low amounts of harmful radiation. But they only account for 6 percent of Milky Way stars and are much shorter lived. In the search for habitable planets, K dwarf stars could be just right, though. Not too rare they have 40 billion year lifetimes, much longer than the Sun. With a relatively wide habitable zone they produce only modest amounts of harmful radiation. These Goldilocks stars account for about 13 percent of the stars of the Milky Way.

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[bystander]

viewtopic.php?t=40183

[Boomer12k]

Something generally not thought of when thinking of Life Zones.... too much radiation... when we look at the night sky what we don't see... would kill us...

:---[===] *

[isoparix]

How is a Goldilock Zone temperature calculated? If it's on the basis of 'black-body' radiation absorption/emission balance, then I think the Earth would be well outside the zone. I thought the temperature of an earth-sized lump of iron would be -15 C. No liquid water there... Our occupancy comes from the our having just the right ampunt of a greenhouse atmosphere to alter that radiation balance and being in the right place. Or is that factored in to these sums already?

[orin stepanek]

I think I will never know what size sun we should be looking for! I would however; trust in what we are user to; and that would be a Sol sized Sun!

[cosmic_ray]

x-ray radiation or gamma rays even, is not so bad, unless it is extremely energetic; that is because the atmosphere totally blocks x-rays and gamma rays.

[FormerAstroSquib]

Can someone explain x-ray irradiance and why smaller K and M stars give off more than the sun? I remember black body curves and don't understand why a smaller star would give off more. Thanks!

[Cousin Ricky]

They [M dwarf stars] are also seen to live long (100 billion years or so) ...

Can I get pedantic here? M dwarf stars are not seen to live 100 billion years, because the universe isn’t yet that old. They are projected to live that long, and I have confidence that astrophysicists know what they are talking about, but they haven’t actually seen it yet.

[orin stepanek]

They [M dwarf stars] are also seen to live long (100 billion years or so) ...

Can I get pedantic here? M dwarf stars are not seen to live 100 billion years, because the universe isn’t yet that old. They are projected to live that long, and I have confidence that astrophysicists know what they are talking about, but they haven’t actually seen it yet.

Good observation; I wonder if that was a typo?

[TheZuke!]

Well, there goes my plans for a vacation home orbiting Proxima Centauri!

[neufer]

Goldilocks Zones and Stars

Explanation: The Goldilocks zone is the habitable zone around a star where it's not too hot and not too cold for liquid water to exist on the surface of orbiting planets. This intriguing infographic includes relative sizes of those zones for yellow G stars like the Sun, along with orange K dwarf stars and red M dwarf stars, both cooler and fainter than the Sun. M stars (top) have small, close-in Goldilocks zones. They are also seen to live long (100 billion years or so) and are very abundant, making up about 73 percent of the stars in the Milky Way. Still, they have very active magnetic fields and may produce too much radiation harmful to life, with an estimated X-ray irradiance 400 times the quiet Sun.

Goldilocks and the Three Bears

<<Folklorists Iona and Peter Opie point out in The Classic Fairy Tales (1999) that [Goldilocks and the Three Bears] has a "partial analogue" in "Snow White": the lost princess enters the dwarfs' house, tastes their food, and falls asleep in one of their beds. In a manner similar to the three bears, the dwarfs cry, "Someone's been sitting in my chair!", "Someone's been eating off my plate!", and "Someone's been sleeping in my bed!" The Opies also point to similarities in a Norwegian tale about a princess who takes refuge in a cave inhabited by three Russian princes dressed in bearskins. She eats their food and hides under a bed.

Maria Tatar, in The Annotated Classic Fairy Tales (2002), notes that [Goldilocks and the Three Bears] is sometimes viewed as a cautionary tale that imparts a lesson about the hazards of wandering off and exploring unknown territory.>>

[BobStein-VisiBone]

Can someone explain x-ray irradiance and why smaller K and M stars give off more than the sun? I remember black body curves and don't understand why a smaller star would give off more. Thanks!

I wondered this too. Those X-rays must not be part of the star's thermal radiation.

Red dwarfs are known to be less stable than sun-like stars. From a Gizmodo article the study of M9 dwarf TVLM 513-46546 "...suggests that planets in orbit around red dwarfs may be subject to tremendously powerful and frequent solar flares..."

The astronomers aren’t sure if M9 dwarf TVLM 513-46546 is an anomaly, or if it’s an example of an entire class of stormy stars. Looking ahead, they plan on studying similar stars to find out.

That 2015 study says "the emission is due to the synchrotron process."

The synchrotron process occurs "when high-energy particles are in acceleration, including electrons forced to travel in a curved path by a magnetic field... Relativistic length contraction ... accelerates the electrons into the X-ray range."

I get the impression the theory on this is new and unsettled. Flares expected.

[Chris Peterson]

Something generally not thought of when thinking of Life Zones.... too much radiation... when we look at the night sky what we don't see... would kill us...

Bad for complex life, almost certainly. But perhaps not for simple life (which is what I suspect the Universe is full of). Indeed, for simple life, a high ionizing radiation environment might be a good thing.

[neufer]

Something generally not thought of when thinking of Life Zones.... too much radiation...
when we look at the night sky what we don't see... would kill us...

Bad for complex life, almost certainly.

But perhaps not for simple life (which is what I suspect the Universe is full of).

Indeed, for simple life, a high ionizing radiation environment might be a good thing.

---

For simple nerd life (which is what I suspect the Asterisk* is full of)
a high ionizing radiation environment might also be a good thing:

Following his accidental exposure to gamma rays during the detonation of an experimental bomb, Dr. Robert Bruce Banner, a physically weak, socially withdrawn, and emotionally reserved physicist is physically transformed into the Hulk when subjected to emotional stress, at or against his will, often leading to destructive rampages and conflicts that complicate Banner's civilian life.

Peter Parker, a high school student from Queens with "obsessions with rejection, inadequacy, and loneliness" acquires spider-related abilities after a bite from a radioactive spider; these include clinging to surfaces, shooting spider-webs from wrist-mounted devices, and detecting danger with his "spider-sense".

Art Neuendorffer (a physically weak, socially withdrawn, & emotionally reserved physicist)

[TheZuke!]

Peter Parker, a high school student from Queens with "obsessions with rejection, inadequacy, and loneliness" acquires spider-related abilities after a bite from a radioactive spider; these include clinging to surfaces, shooting spider-webs from wrist-mounted devices, and detecting danger with his "spider-sense".[/b]

Art Neuendorffer (a physically weak, socially withdrawn, & emotionally reserved physicist)
[/quote]

Queens?
I don't recall Peter Parker or Spiderman speaking with a Queens accent or dialect.

Smart alecky, yes, Queens, no.

[neufer]

I don't recall Peter Parker or Spiderman speaking with a Queens accent or dialect.

Smart alecky, yes, Queens, no.

A high ionizing radiation environment might be a good thing for TheZuke!
(A trip on the Peter Parker Space Probe, perhaps?)

[geoffrey.landis]

Just for context, it would have been nice if it included the Goldilocks zone around F stars, about 3% of the stellar population, and 2-4 billion year lifetimes.
The wild card for F stars will be the UV. I will expect that the UV will be attenuated by atmospheric ozone in a planet of a F star, but that would have to be determined.

[TheOtherBruce]

Can someone explain x-ray irradiance and why smaller K and M stars give off more than the sun? I remember black body curves and don't understand why a smaller star would give off more. Thanks!

One important distinction would be if that figure is for the absolute X-ray output of the star, or the average X-ray flux at the distance of the Goldilocks Zone. The zone is a lot smaller and closer to the star if it's K or M class due to the Inverse-Square Law, so the amount of X-rays at that reduced distance would also increase.

[Chris Peterson]

Can someone explain x-ray irradiance and why smaller K and M stars give off more than the sun? I remember black body curves and don't understand why a smaller star would give off more. Thanks!

One important distinction would be if that figure is for the absolute X-ray output of the star, or the average X-ray flux at the distance of the Goldilocks Zone. The zone is a lot smaller and closer to the star if it's K or M class due to the Inverse-Square Law, so the amount of X-rays at that reduced distance would also increase.

Irradiance is a measure of incident radiation, not emitted radiation (which would be radiant intensity). So it's almost certainly a reference to the flux in the Goldilocks zone

[KKJdrunkenmonkey]

Wow guys, inflate numbers to make your point much?

If M stars are 73%, K are 13%, and G are 6%, then assuming that we're talking about ratios of common they are the graphic should only have 1 G (to set the standard), 2 K (not 3, since Ks are about twice as common as Gs), and 12 M (not 10). Even if you're using something other than how common G stars are as your baseline, the ratios are still off.

While this might sound pedantic to some, I'd like to point out that in the era of fake news any and all falsified info in an attempt to reinforce a point should be called out and fought against. Graphics like this exist solely to be easily consumed, which means the chart is far more likely to be shared and viewed than the text is to be read. I hope someone at NASA reads this and fixes it, but I'm not holding my breath.

[Chris Peterson]

Wow guys, inflate numbers to make your point much?

If M stars are 73%, K are 13%, and G are 6%, then assuming that we're talking about ratios of common they are the graphic should only have 1 G (to set the standard), 2 K (not 3, since Ks are about twice as common as Gs), and 12 M (not 10). Even if you're using something other than how common G stars are as your baseline, the ratios are still off.

While this might sound pedantic to some, I'd like to point out that in the era of fake news any and all falsified info in an attempt to reinforce a point should be called out and fought against. Graphics like this exist solely to be easily consumed, which means the chart is far more likely to be shared and viewed than the text is to be read. I hope someone at NASA reads this and fixes it, but I'm not holding my breath.

The problem here may not be the graphic at all, but a disconnect between the caption and the graphic. The numbers in the caption did not come from the release text accompanying the graphic. Indeed, the only number offered in the release is that K stars are three times more abundant than G stars- which is consistent with the graphic. And looking around a little, there's a pretty significant range of numbers for stellar populations. I suspect the chart accurately represents the stellar population counts as they are presented in this particular work. No paper is online yet to get more information.

[KKJdrunkenmonkey]

The problem here may not be the graphic at all, but a disconnect between the caption and the graphic. The numbers in the caption did not come from the release text accompanying the graphic. Indeed, the only number offered in the release is that K stars are three times more abundant than G stars- which is consistent with the graphic.

I appreciate your response. However, there is one more number in the release text which you missed: "stars like our Sun represent only about 10% of the Milky Way population."

So, if our sun is 10% of the population, and K stars are three times more abundant, then the graphic is good so far for 40% of the galaxy's population. So... where did the 10 M stars come from? At most it should have been 6 to make 100%.

Regardless of how you look at it nothing seems to line up. The graphic and its accompanying release text do not agree with one another, and whoever wrote the caption appears to have not actually looked at either of those things before plowing ahead with their piece. Not impressed.

[Chris Peterson]

The problem here may not be the graphic at all, but a disconnect between the caption and the graphic. The numbers in the caption did not come from the release text accompanying the graphic. Indeed, the only number offered in the release is that K stars are three times more abundant than G stars- which is consistent with the graphic.

I appreciate your response. However, there is one more number in the release text which you missed: "stars like our Sun represent only about 10% of the Milky Way population."

So, if our sun is 10% of the population, and K stars are three times more abundant, then the graphic is good so far for 40% of the galaxy's population. So... where did the 10 M stars come from? At most it should have been 6 to make 100%.

Regardless of how you look at it nothing seems to line up. The graphic and its accompanying release text do not agree with one another, and whoever wrote the caption appears to have not actually looked at either of those things before plowing ahead with their piece. Not impressed.

It could well be that something is wrong with the graphic. But the fact remains that population figures for different stellar classes are all over the board, depending on source (and even on definition, which varies). It would be interesting to see the paper which is being presented for clarification. It's certainly not uncommon to encounter poorly written press releases that don't accurately reflect the actual work (not an excuse, just an observation).

[Ann]

The problem here may not be the graphic at all, but a disconnect between the caption and the graphic. The numbers in the caption did not come from the release text accompanying the graphic. Indeed, the only number offered in the release is that K stars are three times more abundant than G stars- which is consistent with the graphic.

I appreciate your response. However, there is one more number in the release text which you missed: "stars like our Sun represent only about 10% of the Milky Way population."

So, if our sun is 10% of the population, and K stars are three times more abundant, then the graphic is good so far for 40% of the galaxy's population. So... where did the 10 M stars come from? At most it should have been 6 to make 100%.

Regardless of how you look at it nothing seems to line up. The graphic and its accompanying release text do not agree with one another, and whoever wrote the caption appears to have not actually looked at either of those things before plowing ahead with their piece. Not impressed.

According to Wikipedia, G-type main sequence stars make up 7,6% of all main sequence stars in the Milky Way. K-type main sequence stars make up 12.1% of all main sequence stars, and M-type main sequence stars make up 76.45% of all main sequence stars.

F-type main sequence stars make up 3% of the main sequence stars. A-type main sequence stars (like for example Sirius) make up 0.3% of all main sequence stars in the Milky Way. Main sequence B-type stars (like Alkaid, the end star of the handle of the Big Dipper) make up 0.13% of all main sequence stars. Main sequence O-type stars, like 10 Lacertae, make up ~0.00003% of all main sequence stars in the Milky Way.

Note that not all G-type main sequence stars are going to resemble the Sun. The G-type classification includes stars from spectral class G0 to spectral class G9, and since the Sun is a G2V-type star, it seems likely that the majority of G-type stars will be somewhat cooler and dimmer than the Sun.

Not all stars are main sequence stars. A non-negligible population is made up of white dwarfs. Another type of star that is numerous enough to be taken into account is the modest red giant stars like Pollux or Dubhe.

Note that the figures are not exact, because we can't know these numbers for sure. But we do have a very reasonable idea.

Ann

[KKJdrunkenmonkey]

According to Wikipedia...

The numbers on Wikipedia have a tag on them indicating that they know they aren't accurate. Hovering over the superscript e at the end of any of the sentences claiming a percentage reveals this: "These proportions are fractions of stars brighter than absolute magnitude 16; lowering this limit will render earlier types even rarer, whereas generally adding only to the M class."

Thanks Chris. Your explanation is making a lot more sense. Apparently the people who make the graphics and the people who write the text should consider talking to one another next time.