APOD: In the Center of the Trifid Nebula (2023 Jun 05)

[APOD Robot]

In the Center of the Trifid Nebula

Explanation: What's happening at the center of the Trifid Nebula? Three prominent dust lanes that give the Trifid its name all come together. Mountains of opaque dust appear near the bottom, while other dark filaments of dust are visible threaded throughout the nebula. A single massive star visible near the center causes much of the Trifid's glow. The Trifid, cataloged as M20, is only about 300,000 years old, making it among the youngest emission nebulas known. The star forming nebula lies about 9,000 light years away toward the constellation of the Archer (Sagittarius). The region pictured here spans about 20 light years.

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[Ann]

In the Center of the Trifid Nebula (detail). Credit: Martin Pugh

I like the portrait of the inner workings of the Trifid Nebula, and how the dust reflects the blue color of the extremely young and hot stars. All the dust in the center of the Trifid Nebula is itself a testament to how young this nebula is, since the intense ultraviolet radiation and the harsh wind from the newborn massive stars will quickly disperse the dust (and, as David Malin once told me, "cook" the dust particles to a smaller size).

I cropped the image because the neon-cyan color of the reflection part of the Trifid Nebula in this image clashes with my sense of aesthetics.

Ann

[johnnydeep]

Too many questions for a Monday:

1. Wikipedia thinks the nebula is 4200 ly away (with a reference from 2018), and this APOD thinks it's 9000?
2. The distance should be roughly the same as the distance to the central star (or stars) powering the glow, no?
3. How do they measure the age of a nebula?
4. Wikipedia talks about a "jet" (somewhere), that points to the central star. Where is that jet?
5. And where is that central star?
6. Trifid should really be Quatrofid, now that better imaging clearly shows 4 "lobes" not 3!

[Chris Peterson]

Too many questions for a Monday:

1. Wikipedia thinks the nebula is 4200 ly away (with a reference from 2018), and this APOD thinks it's 9000?
2. The distance should be roughly the same as the distance to the central star (or stars) powering the glow, no?
3. How do they measure the age of a nebula?
4. Wikipedia talks about a "jet" (somewhere), that points to the central star. Where is that jet?
5. And where is that central star?
6. Trifid should really be Quatrofid, now that better imaging clearly shows 4 "lobes" not 3!

For HD 164492A, one of the bright stars that has formed in the nebula, the Gaia DR3 catalog (the best distance reference we have) gives a parallax of 0.684 ± 0.048 mas, which translates to a distance of 1462 pc or 4772 ly, with about a 1% uncertainty.

This is not a planetary nebula, so it has no "central star" as such. The ionization is probably created by a number of bright stars in the cluster.

[calahnst@sisqtel.net]

The article says there is one massive star in the middle.
If expanded the view shows actually 4 stars. 2 Massive and 2 smaller or more distant.

[Chris Peterson]

The article says there is one massive star in the middle.
If expanded the view shows actually 4 stars. 2 Massive and 2 smaller or more distant.

HD 164492A is the more northern (towards the top) of the bright pair seen in the central part of the nebula in this image. That is the source that is credited with producing the largest share of UV which is ionizing the surrounding hydrogen.

[Ann]

Too many questions for a Monday:

1. Wikipedia thinks the nebula is 4200 ly away (with a reference from 2018), and this APOD thinks it's 9000?
2. The distance should be roughly the same as the distance to the central star (or stars) powering the glow, no?
3. How do they measure the age of a nebula?
4. Wikipedia talks about a "jet" (somewhere), that points to the central star. Where is that jet?
5. And where is that central star?
6. Trifid should really be Quatrofid, now that better imaging clearly shows 4 "lobes" not 3!

For HD 164492A, one of the bright stars that has formed in the nebula, the Gaia DR3 catalog (the best distance reference we have) gives a parallax of 0.684 ± 0.048 mas, which translates to a distance of 1462 pc or 4772 ly, with about a 1% uncertainty.

This is not a planetary nebula, so it has no "central star" as such. The ionization is probably created by a number of bright stars in the cluster.

Chris has given you the best available distance estimate to the Trifid Nebula.

I agree with Chris that we can't talk about a "central star" of the Trifid Nebula, but for all of that, there is a star in the Trifid Nebula (just as there is in the Orion Nebula) that provides most of the ionization of the nebula. In the case of the Trifid Nebula, the star is HD 164492A, a star of spectral class O7.5V, and in the case of the Orion Nebula, the star is Theta 1 Orionis C, a star of spectral class O6V.

How do they measure the age of the nebula? Well, groan. That's a tricky question. One clue is whether the nebula is small, bright, compact and highly ionized, or if it is large, faint, expansive and not so highly ionized. Compare the appearance of the Orion Nebula and the Lambda Orionis nebula, two nebulas that are probably at more or less the same distance from us:

To me it is obvious that small bright Orion Nebula must be younger than large faint Lambda Orionis Nebula, because nebulas disperse over time.

Another clue is the color-magnitude diagram of the cluster of stars inside the nebula. Are there any bright red stars in the cluster? Are the brightest blue stars still on the main sequence? And have the K- and M-type orange and red dwarfs arrived on the main sequence yet?

Zero-age main sequence. The very young massive blue stars have arrived on the main sequence, but the small red stars have not. There are no red giants.

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Color-Magnitude diagrams for various open clusters. The youngest clusters have long blue main sequences and no red giants. As the clusters age, they lose more and more of their bright blue stars and get red giants instead. Credit: Penn State Astronomy & Astrophysics

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The youngest of the open clusters in the Penn State University comparison in NGC 2362. However, while this cluster has bright blue stars and no red giants, it is old enough that it has blown away its own birth nebula.

NGC 2362. Credit: Langkawi National Observatory)

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As for the jets in the Trifid Nebula, there are two somewhat famous jets in it. But these two jets don't point at the hot ionizing stars of the Trifid Nebula:

Jets in the Trifid Nebula. Credit: James Long & the ESA/ESO/NASA Photoshop FITS Liberator

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You can see the two jets at lower left. Credit: International Gemini Observatory/Ingrid Braul, Southlands Elementary, Vancouver BC

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The jets emerge from a fat pillar, similar to the Pillars of Creation in the Eagle Nebula. The pillar in the Trifid points, more or less, at the cluster of hot stars in the center of the nebula.

And where is the brightest star of the Trifid Nebula? Well, I'd say it is probably the brightest star is this (rather old) ESA/Hubble image:

The central star cluster of the Trifid Nebula. Credit: ESA/Hubble.

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And I agree with you that the Trifid Nebula should be called the Quatrofid, since it clearly has four lobes and not three!

Ann

[orin stepanek]

JMO!

[johnnydeep]

Thanks for the explanations guys and gals!

[Ann]

Thanks for the explanations guys and gals!

One more thing, Johnny. How do we know that the Lagoon Nebula is older than the Trifid Nebula? It is because the Lagoon Nebula is a multi-generation nebula, where star formation has been going on for at least 15 million years, whereas the Trifid has very recently (300,000 years ago?) given birth to its first stars.


Anyway, the Lagoon nebula contains an older cluster, NGC 6530, while at the same time star formation is still going on in another part of the Lagoon Nebula, in the central bright part of it known as the Hourglass Nebula.

The 15 million year old cluster NGC 6530 and the very young Hourglass Nebula in the Lagoon. Image credit: Juanma Riviero

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The star Herschel 36 in the Hourglass Nebula in the center of the Lagoon Nebula. The fact that the star is involved with so much dust proves that it is very young. Credit: A. Caulet (ST-ECF, ESA) and NASA

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Just as it is with the very young star Herschel 36 in the Lagoon, the young stars in the Trifid Nebula are involved with a lot of dust. And in my opinion, the presence of a large blue reflection nebula immediately adjacent to the red emission nebula is in itself a clear sign that the Trifid nebula must be young. The blue reflection nebula means that there is dust here that the hot bright Trifid Nebula stars have not had time to blow away.

A blue reflection nebula adjacent to the red emission nebula of the Trifid Nebula. Image credit: Bart Delsaert.

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The star cluster in the center of the Trifid Nebula. There is a lot of dust here. Image credit: ESA/Hubble.

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It is actually quite unusual to see blue reflection nebulas and red emission nebulas close together, and we have very good reasons reason to believe that such nebulas are young:

The young O-type star 15 Mon is surrounded by red emission nebulosity, but there is also a blue reflection nebula right next to it. Image credit: Astrovetteman at Cloudy Nights.

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Runaway star AE Aurigae has blundered into a cloud of gas and dust while on its headlong rush through the galaxy. During its brief visit to the cloud, AE Aurigae has ionized the gas and made it glow red, and made its strands of dust glow blue in reflection of AE Aurigae's own blue hue. Image credit: Adam Block, Mt. Lemmon SkyCenter, Univ. Arizona

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Ann

[johnnydeep]

Thx, Ann.