APOD: The Medusa Nebula (2024 Nov 22)

[APOD Robot]

The Medusa Nebula

Explanation: Braided and serpentine filaments of glowing gas suggest this nebula's popular name, The Medusa Nebula. Also known as Abell 21, this Medusa is an old planetary nebula some 1,500 light-years away in the constellation Gemini. Like its mythological namesake, the nebula is associated with a dramatic transformation. The planetary nebula phase represents a final stage in the evolution of low mass stars like the sun as they transform themselves from red giants to hot white dwarf stars and in the process shrug off their outer layers. Ultraviolet radiation from the hot star powers the nebular glow. The Medusa's transforming star is the faint one near the center of the overall bright crescent shape. In this deep telescopic view, fainter filaments clearly extend below and to the left. The Medusa Nebula is estimated to be over 4 light-years across.

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[RC Davison]

It would be very helpful to add an overlay pointing out an object referenced in the accompanying text, as sometimes it’s difficult to know which faint star is the right one when there are a group of stars in the general area. In today’s post the reference to the crescent “… faint one near the center of the overall bright crescent shape.” Isn’t very helpful since the nebula is diffuse and not well defined.

APOD is still the first thing I check every morning - it’s always a treat! Keep up the great work!

[Ann]

It would be very helpful to add an overlay pointing out an object referenced in the accompanying text, as sometimes it’s difficult to know which faint star is the right one when there are a group of stars in the general area. In today’s post the reference to the crescent “… faint one near the center of the overall bright crescent shape.” Isn’t very helpful since the nebula is diffuse and not well defined.

APOD is still the first thing I check every morning - it’s always a treat! Keep up the great work!

If the image is a reasonably good RGB ("true color") one, and you want to find the central star of a planetary nebula, then you should always look for a blue-looking star near the center of the nebula. If the central star has moved relative to the nebula, which can happen, then it might become to hard to find it. But that is not the case for the Medusa Nebula.

APOD 22 November 2024 annotated.png

As you can see, the star that I have labeled the central star is both blue and quite centrally located.

I also placed the Earth on one side of the Medusa Nebula and Proxima Centauri, the nearest star to the Earth after the Sun, on both sides of the Medusa Nebula, to show the true size of the nebula (four light-years). Of course, I'm not sure that I put Proxima Centauri in the proper place!

Back to the blue stars of the APOD. The stars are all the same shade of blue. In a very good RGB photo, the central star of a planetary nebula should be bluer than any other star in the picture, unless the central star is strongly reddened by dust. That is not the case for the central star in the Medusa Nebula. And indeed, in a picture of the Medusa Nebula by ESO, the central star really looks bluer than any other star in the image, even though the picture is (mostly) a narrowband one.

Medusa Nebula annotated ESO.png

The Medusa Nebula with a very blue-looking central star.
Credit: ESO.

Note the cute little background galaxy too. In reality this galaxy may not be so little at all. It has a very strong bar.

Ann

[RC Davison]

Thank you, Ann.

I suspected the blue star because it did seem to be closest to the perceived center, but I wasn't sure. Is the blue coloration due to the higher surface temperature of the white dwarf or is the dust and gas from the nebula filtering out the longer wavelengths?

[Ann]

Thank you, Ann.

I suspected the blue star because it did seem to be closest to the perceived center, but I wasn't sure. Is the blue coloration due to the higher surface temperature of the white dwarf or is the dust and gas from the nebula filtering out the longer wavelengths?

The blue color of the central stars of planetary nebulas is due to the very high temperature of these stars.

White dwarfs - and central stars of planetary nebulas are white dwarfs - cool over time and eventually become too cold to emit any light at all, but the central stars of planetary nebulas are young. They are still very hot, hotter than any "normal" stars, hotter even than O-type stars like Alnitak in Orion's Belt, 10 Lacertae, AE Aurigae or Lambda Orionis. "Normal" O-type stars are rarely hotter than 40,000 K, and they are often cooler than that. But central stars of planetary nebulas are often hotter than 50,000 K, and they can be as hot as 100,000 K or even hotter. But the hottest central stars of planetary nebulas are the youngest, and they may be shrouded in dust, so that they don't look blue.

Let's look at two examples:

NGC 7027. This is a very young planetary nebula whose central star has recently had an outburst, creating a very asymmetric dust cloud. The central star is hidden by dust and can be seen in this image only because both optical and infrared filters were used to observe it. Credit: ESA/Hubble.

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IC 1295. The small central star is centrally located and very blue, bluer than any other star in the surrounding field, with the possible exception of a very blue-looking star at upper right. Credit: Adam Block.

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Ann

[Christian G.]

But the hottest central stars of planetary nebulas are the youngest

But don't these central stars heat up as they contract over time, rather than being at their hottest temperature at the start of the planetary nebula process? (up untill reaching white dwarf stage of course, when they begin to slowly cool down for good)
The numbers below vary according to sources but here's a tentative sample where increasing temperatures apparently correlates with increasing age:
Spirograph Nebula age, 1400 years, central star temperature, 40 000 K

spirograph.jpg

Cat's Eye Nebula age, 2000 years, central star, 80 000 K (and not done getting hotter apparently!)

Catseye.jpg

Ring Nebula age, 3400 years, central star, 120 000 K

Ring.JPG

[Chris Peterson]

But the hottest central stars of planetary nebulas are the youngest

But don't these central stars heat up as they contract over time, rather than being at their hottest temperature at the start of the planetary nebula process? (up untill reaching white dwarf stage of course, when they begin to slowly cool down for good)

Indeed, the star is at its coolest when it first starts shedding its outer layers, and grows hotter as the planetary nebula evolves (with the nebula getting brighter as the UV intensity increases), until it reaches its peak temperature near the end of fusion (and its transition to a white dwarf) and then begins its long, long slow cooling.

[Ann]

But the hottest central stars of planetary nebulas are the youngest

But don't these central stars heat up as they contract over time, rather than being at their hottest temperature at the start of the planetary nebula process? (up untill reaching white dwarf stage of course, when they begin to slowly cool down for good)
The numbers below vary according to sources but here's a tentative sample where increasing temperatures apparently correlates with increasing age:
Spirograph Nebula age, 1400 years, central star temperature, 40 000 K
spirograph.jpg


Cat's Eye Nebula age, 2000 years, central star, 80 000 K (and not done getting hotter apparently!)
Catseye.jpg


Ring Nebula age, 3400 years, central star, 120 000 K
Ring.JPG

That was careless of me, Christian. You are right, and I was wrong.

Ann