APOD: The Fainting of Betelgeuse (2020 Jan 02)

[APOD Robot]

The Fainting of Betelgeuse

Explanation: Begirt with many a blazing star, Orion the Hunter is one of the most recognizable constellations. In this night skyscape the Hunter's stars rise in the northern hemisphere's winter sky on December 30, 2019, tangled in bare trees near Newnan, Georgia, USA. Red super giant star Betelgeuse stands out in yellowish hues at Orion's shoulder left of center, but it no longer so strongly rivals the blue supergiant star Rigel at the Hunter's foot. In fact, skygazers around planet Earth can see a strikingly fainter Betelgeuse now, its brightness fading by more than half in the final months of 2019. Betelgeuse has long been known to be a variable star, changing its brightness in multiple cycles with approximate short and long term periods of hundreds of days to many years. The star is now close to its faintest since photometric measurements in 1926/27, likely due in part to a near coincidence in the minimum of short and long term cycles. Betelgeuse is also recognized as a nearby red supergiant star that will end its life in a core collapse supernova explosion sometime in the next 1,000 years, though that cosmic cataclysm will take place a safe 700 light-years or so from our fair planet.

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[BDanielMayfield]

Betelgeuse is also recognized as a nearby red supergiant star that will end its life in a core collapse supernova explosion sometime in the next 1,000 years, though that cosmic cataclysm will take place a safe 700 light-years or so from our fair planet.

Is that 1,000 years figure a mistake? Yesterday's discussion indicated sometime in the next 100,000 years.

[Ann]

Click to view full size image

The fainting of Betelgeuse. Photo: Jimmy Westlake.

Oh wow, that's just such a gorgeous image!!!

I'm reminded of people who have deciduous trees in their gardens and choose to string colorful Christmas lights on the naked branches of those trees. That's what this image looks like to me, a totally festive Christmas light show among the naked branches of trees!

The colors of the stars are very saturated, and I love it. Did I say I love it? Note that the hues of the fainter stars are particularly saturated, which is due to the fact that the fainter stars are not overexposed. Interestingly, Betelgeuse itself is pretty pale in color, even though it really does look strangely faint.

Many other stars look very red. Note in the picture at left this group of stars near the top of the APOD. See that very red star at the bottom of this group of stars? That's W Orionis, HD 32736. It's a carbon star of spectral class N5, and it is so red that if you could place it next to Betelgeuse in the sky, Betelgeuse itself would look very nearly stark white in color! Check out this star through a telescope if you get the chance.


Also note the pretty pairing of a blue and a fainter orange star at the top of this group of stars. The blue star is π5 Orionis (pi5 Ori), and it is a very nice blue B2III-type star. The orange star next to it is 5 Ori, HD 31139, and it is an M1III-type star and relatively red. But the color of 5 Ori is nothing compared with the hue of W Orionis!

Ann

[Ann]

Betelgeuse is also recognized as a nearby red supergiant star that will end its life in a core collapse supernova explosion sometime in the next 1,000 years, though that cosmic cataclysm will take place a safe 700 light-years or so from our fair planet.

Is that 1,000 years figure a mistake? Yesterday's discussion indicated sometime in the next 100,000 years.

Click to view full size image

Nuclear reactions in the Sun's core produce light and other types of radiation.
As light photons leave the core, they run into electrons and atomic nuclei,
scattering off each one. These interactions cause photons to take,
on average, 200,000 years to move from the Sun's core to its surface.
Illustration: Roen Kelly, Astronomy Magazine.

The 1,000 years figure is definitely a mistake! Yes, Betelgeuse could go in 1,000 years. Heck, it could go tomorrow, for all we know.

But it might just as easily last for another 100,000 years, or maybe even longer. After all, it is the condition of the core of Betelgeuse that determines when it will explode. And the core of any star is hidden from us behind thick opaque gases. The reason why the gases are opaque is that the photons from the core keep scattering off electrons and atomic nuclei on their way to the surface of the star.

Ann

[Pannick]

But if it happened 699 years and 6 months ago we would only know about this year - wouldn't we? Help!

[BDanielMayfield]

The 1,000 years figure is definitely a mistake!

I thought so too, but didn't want to be so blunt about it. But I also held out a small hope that maybe it was true somehow. Darn.

Being normally among the brighter stars, Betelgeuse would be too bright for the Gaia mission to study. But if it fades enough (and for long enough) maybe Gaia can nail down its distance and motion

Bruce

[BDanielMayfield]

Being normally among the brighter stars, Betelgeuse would be too bright for the Gaia mission to study. But if it fades enough (and for long enough) maybe Gaia can nail down its distance and motion

After looking up info on Gaia I now see that it doesn't have the brightness limitation that I thought it did, for it can collect data on all stars down to Mag 20. I must have gotten this mission confused with another.

Bruce

[BDanielMayfield]

Being normally among the brighter stars, Betelgeuse would be too bright for the Gaia mission to study. But if it fades enough (and for long enough) maybe Gaia can nail down its distance and motion

After looking up info on Gaia I now see that it doesn't have the brightness limitation that I thought it did, for it can collect data on all stars down to Mag 20. I must have gotten this mission confused with another.

However, I now find that Gaia did originally have an overly bright star limitation, but work-arounds are expected to solve this limitation.

Although the European Space Agency's current Gaia mission was not expected to produce good results for stars brighter than the approximately V=6 saturation limit of the mission's instruments,[86] actual operation has shown good performance on objects to about magnitude +3. Forced observations of brighter stars mean that final results should be available for all bright stars and a parallax for Betelgeuse will be published an order of magnitude more accurate than currently available.[87]

[orin stepanek]

I love the Orion Constellation & it's beauty in the night sky!

[Astronymus]

But if it happened 699 years and 6 months ago we would only know about this year - wouldn't we? Help!

Roughly around that, yes. We get a few hours warning though. Neutrinos will escape the star in peak numbers before any light or shockwave can.

[Chris Peterson]

But if it happened 699 years and 6 months ago we would only know about this year - wouldn't we? Help!

The event is defined by its observation. It happens when the first radiation reaches us. That's when we'll know about it. That will be t=0.

[neufer]

Betelgeuse is also recognized as a nearby red supergiant star that will end its life in a core collapse supernova explosion sometime in the next 1,000 years, though that cosmic cataclysm will take place a safe 700 light-years or so from our fair planet.

Is that 1,000 years figure a mistake? Yesterday's discussion indicated sometime in the next 100,000 years.

The 1,000 years figure is definitely a mistake! Yes, Betelgeuse could go in 1,000 years. Heck, it could go tomorrow, for all we know.

But it might just as easily last for another 100,000 years, or maybe even longer. After all, it is the condition of the core of Betelgeuse that determines when it will explode. And the core of any star is hidden from us behind thick opaque gases. The reason why the gases are opaque is that the photons from the core keep scattering off electrons and atomic nuclei on their way to the surface of the star.

• You are right about the 1,000 years figure, Ann.
  
  However, red giants don't have much of a radiative zone
  and core/burning shell changes may not take 171,000 years to signal us.

<<A radiation zone, or radiative region is a layer of a star's interior where energy is primarily transported toward the exterior by means of radiative diffusion and thermal conduction, rather than by convection. Energy travels through the radiation zone in the form of electromagnetic radiation as photons. Matter in a radiation zone is so dense that photons can travel only a short distance before they are absorbed or scattered by another particle, gradually shifting to longer wavelength as they do so. For this reason, it takes an average of 171,000 years for gamma rays from the core of the Sun to leave the radiation zone. Over this range, the temperature of the plasma drops from 15 million K near the core down to 1.5 million K at the base of the convection zone.>>

An illustration of the structure of the Sun and a red giant star, showing their convective zones. These are the granular zones in the outer layers of the stars.

---

<<A convection zone, convective zone or convective region of a star is a layer which is unstable to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiation and conduction.

Stellar convection consists of mass movement of plasma within the star which usually forms a circular convection current with the heated plasma ascending and the cooled plasma descending.

If the temperature gradient is steep enough (i. e. the temperature changes rapidly with distance from the center of the star), or if the gas has a very high heat capacity (i. e. its temperature changes relatively slowly as it expands) then the rising parcel of gas will remain warmer and less dense than its new surroundings even after expanding and cooling. Its buoyancy will then cause it to continue to rise. The region of the star in which this happens is the convection zone.

In the most massive stars, the convection zone may reach all the way from the core to the surface. In red giant stars, and particularly during the asymptotic giant branch phase, the surface convection zone varies in depth during the phases of shell burning. This causes dredge-up events, short-lived very deep convection zones that transport fusion products to the surface of the star.>>

[neufer]

"Begirt with many a blazing star"

https://apod.nasa.gov/apod/ap200102.html
https://apod.nasa.gov/apod/ap120119.html

More like begirt with many a bare branch!

[Fred the Cat]

Yes, time equal to zero should be quite a celestial event.

[neufer]

Astronomical defining of t=0 as lying along our own past light cone

---

But if it happened 699 years and 6 months ago we would only know about this year - wouldn't we? Help!

The event is defined by its observation. It happens when the first radiation reaches us. That's when we'll know about it. That will be t=0.

Yes, time equal to zero should be quite a celestial event.

For all sorts of pragmatic reasons it is
useful in almost all cases to define
t=0 as lying along our own past light cone.

(General relativity allows us to arbitrarily set space-time coordinates.)

[Chris Peterson]

(General relativity allows us to arbitrarily set space-time coordinates.)

I think that's more accurately seen as a feature of special relativity.

[TheZuke!]

The resident "know it all" in my workplace has said that when Betelgeuse explodes, its cloud will eventually engulf our solar system reducing visible stars to about one third.

Has anyone else heard/read of that?

[Chris Peterson]

The resident "know it all" in my workplace has said that when Betelgeuse explodes, its cloud will eventually engulf our solar system reducing visible stars to about one third.

Has anyone else heard/read of that?

Not even close. Even assuming that the material isn't dissipated unevenly by other forces, imagine the material of a star spread into a volume of about a billion cubic light years. That's a very hard vacuum.

[geckzilla]

The resident "know it all" in my workplace has said that when Betelgeuse explodes, its cloud will eventually engulf our solar system reducing visible stars to about one third.

Has anyone else heard/read of that?

Not even close. Even assuming that the material isn't dissipated unevenly by other forces, imagine the material of a star spread into a volume of about a billion cubic light years. That's a very hard vacuum.

Plus, like... wouldn't it have already happened? Wouldn't we be trapped in dust-obscured darkness by now if every supernova within a few hundred light years darkened the stars? This know-it-all doesn't know enough to select sources that aren't wildly fictitious.

[neufer]

The resident "know it all" in my workplace has said that when Betelgeuse explodes, its cloud will eventually engulf our solar system reducing visible stars to about one third.

Has anyone else heard/read of that?

Not even close. Even assuming that the material isn't dissipated unevenly by other forces, imagine the material of a star spread into a volume of about a billion cubic light years. That's a very hard vacuum.

The issue isn't so much with the initial blasted material as with all the intervening material that is swept up into a relatively thin shock front.

I estimate that it might reach us in about 2 million years (or around the time the Vela Supernova Remnant reaches us). During the limited time we are within a shock front there may be some noticeable obscuration of stars that also lie along the same shock front...especially if the UV from such stars results in local fluorescence.

[Ann]

The resident "know it all" in my workplace has said that when Betelgeuse explodes, its cloud will eventually engulf our solar system reducing visible stars to about one third.

Has anyone else heard/read of that?

Not even close. Even assuming that the material isn't dissipated unevenly by other forces, imagine the material of a star spread into a volume of about a billion cubic light years. That's a very hard vacuum.

The issue isn't so much with the initial blasted material as with all the intervening material that is swept up into a relatively thin shock front.

I estimate that it might reach us in about 2 million years (or around the time the Vela Supernova Remnant reaches us). During the limited time we are within a shock front there may be some noticeable obscuration of stars that also lie along the same shock front...especially if the UV from such stars results in local fluorescence.

Click to view full size image

Vela supernova remnant. Robert Gendler, Roberto Colombari, Digitized Sky Survey.

This is the Vela supernova remnant. You can see thin blue-green lines of glowing OIII, and some thin pink lines of ionized hydrogen.

How opaque are this fronts? My amateur impression is that they wouldn't cause much obscuration in the sky.

Ann

Edit: I love it when Art and Chris start arguing about general and special relativity.

[neufer]

The resident "know it all" in my workplace has said that when Betelgeuse explodes, its cloud will eventually engulf our solar system reducing visible stars to about one third.

Has anyone else heard/read of that?

Not even close. Even assuming that the material isn't dissipated unevenly by other forces, imagine the material of a star spread into a volume of about a billion cubic light years. That's a very hard vacuum.

The issue isn't so much with the initial blasted material as with all the intervening material that is swept up into a relatively thin shock front.

I estimate that it might reach us in about 2 million years (or around the time the Vela Supernova Remnant reaches us). During the limited time we are within a shock front there may be some noticeable obscuration of stars that also lie along the same shock front...especially if the UV from such stars results in local fluorescence.

After rethinking...

The Cygnus Loop is ~130 ly in diameter at an age of around 21,000 years.
Simeis 147 is ~160 ly in diameter at an age of around 40,000 years.

Once SNRs are this large they have already swept up at least a thousand solar masses of interstellar medium.

Every time they double their size after that their mass becomes 8 times larger and so they must then expand 8 times slower.

momentum = MV = R³dR/dt = C₁

R⁴ = C₂ t

R = C₃ t^1/4

Hence, the expansion (after ~200 ly in diameter) soon drops below a sound velocity of ~5 km/s and the shock fronts all dissipate.

(Note: Betelgeuse is already moving away at ~21 km/s so its shock front stops approaching us before dissipating.)

[TheZuke!]

After rethinking...

The Cygnus Loop is ~130 ly in diameter at an age of around 21,000 years.
Simeis 147 is ~160 ly in diameter at an age of around 40,000 years.

Once SNRs are this large they have already swept up at least a thousand solar masses of interstellar medium.

Every time they double their size after that their mass becomes 8 times larger and so they must then expand 8 times slower.

momentum = MV = R³dR/dt = C₁

R⁴ = C₂ t

R = C₃ t^1/4

Hence, the expansion (after ~200 ly in diameter) soon drops below a sound velocity of ~5 km/s and the shock fronts all dissipate.

(Note: Betelgeuse is already moving away at ~21 km/s so its shock front stops approaching us before dissipating.)

Wow!
Thanks!

[Ann]

After rethinking...

The Cygnus Loop is ~130 ly in diameter at an age of around 21,000 years.
Simeis 147 is ~160 ly in diameter at an age of around 40,000 years.

Once SNRs are this large they have already swept up at least a thousand solar masses of interstellar medium.

Every time they double their size after that their mass becomes 8 times larger and so they must then expand 8 times slower.

momentum = MV = R³dR/dt = C₁

R⁴ = C₂ t

R = C₃ t^1/4

Hence, the expansion (after ~200 ly in diameter) soon drops below a sound velocity of ~5 km/s and the shock fronts all dissipate.

(Note: Betelgeuse is already moving away at ~21 km/s so its shock front stops approaching us before dissipating.)

Wow!
Thanks!

Hey, Art, I'm impressed, too. And I could fully understand your reasoning!

On a different note, last night the sky was finally clear enough for me to see Betelgeuse and the entire Orion constellation. Yes, Orion sure looks strange with Betelgeuse so faint! The sky was misty and the seeing was quite poor, and when Betelgeuse was low and Aldebaran was higher, Aldebaran blazed like an orange traffic light of the sky, while Betelgeuse looked wan. Betelgeuse looked equal in brightness to Bellatrix, the "other shoulder" of Orion. Rigel looked "blatant" as Orion's left foot.

Ann

[neufer]

Click to play embedded YouTube video.