APOD: A Sagittarius Triplet (2021 Apr 26)

[APOD Robot]

A Sagittarius Triplet

Explanation: These three bright nebulae are often featured on telescopic tours of the constellation Sagittarius and the crowded starfields of the central Milky Way. In fact, 18th century cosmic tourist Charles Messier cataloged two of them; M8, the large nebula below and right of center, and colorful M20 near the top of the frame. The third emission region includes NGC 6559, left of M8 and separated from the larger nebula by a dark dust lane. All three are stellar nurseries about five thousand light-years or so distant. Over a hundred light-years across the expansive M8 is also known as the Lagoon Nebula. M20's popular moniker is the Trifid. Glowing hydrogen gas creates the dominant red color of the emission nebulae. But for striking contrast, blue hues in the Trifid are due to dust reflected starlight. The broad interstellarscape spans almost 4 degrees or 8 full moons on the sky.

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[Ann]

I love this image! The colors and details are splendid. I have annotated the image.

Here's what the annotations mean. I'm adding information about the parallaxes of all the stars. The smaller the parallax, the farther away is the star, and the intrinsically brighter it is. I leave it to you math-savvy people to figure out how far the different stars are!

1) HD 166464, K0III, B-V 1.05, parallax 12.6303 [0.2449] Foreground star, slightly similar to Pollux.

2) HD 166033 OB, B-V 0.04, parallax 0.7202 [0.0535] Both emission and reflection. Creates nebula IC 1274. Image by R Jay GaBany.

3) HD 166056 B2V, by red ridge nebula NGC 6559. B-V 0.26, parallax 0.6279 [0.0603]. Image from Wikipedia.

4) HD 165921, O7V(n)z+B0:V, B-V 0.08, parallax 0.9082 [0.0563], large rather faint magenta nebula

5) TYC 6842-1630-1, OB, B-V 0.10, parallax 0.7481 [0.0548] Blue reflection nebula created by blue star, close to orange star.

6) 9 Sgr, O4V((f))z, B-V 0.03, parallax 0.8505 [0.0952] Brightest star in the Lagoon Nebula.

7) HD 164492, O7.5V, B-V 0.0, parallax 0.6376 [0.0873] Brightest star in the Trifid nebula. Picture from ESA/Hubble.

8) 4 Sgr, A0 E, B-V -0.03, parallax 8.3677 [0.4521] Foreground star. A bright Vega?


As you can see, most of the stars have really small parallaxes, smaller than one milliarcsecond. This means that these stars and nebulas are much farther away than the nebulas and bright stars in Orion. The parallaxes I have quoted are from Gaia and should be correct, which means we should have a pretty good handle on how far away these stars are by converting parallaxes to light-years.

So, you math people out there, I challenge you! Will you figure out the distances to these stars for me?

Ann

[orin stepanek]

I'll try to work this photo into my background collection!

[Holger Nielsen]

Hello Ann!

You get the distance in parsecs by dividing 1000 with the parallax in mas; and in light years by multiplying with 3.261.
But for you, below are the values, of course using color to group results.

I looked up the parallax for 9 Sagittarii on English Wikipedia: 0.49 ± 0.40 mas (2007). I guess you have a more recent source.

A technical question: I attached the table image by dragging. It looks tiny, can its size by changed?

[bystander]

A technical question: I attached the table image by dragging. It looks tiny, can its size by changed?

Click on it

[Ann]

Hello Ann!

You get the distance in parsecs by dividing 1000 with the parallax in mas; and in light years by multiplying with 3.261.
But for you, below are the values, of course using color to group results.

StarDistances.png

I looked up the parallax for 9 Sagittarii on English Wikipedia: 0.49 ± 0.40 mas (2007). I guess you have a more recent source.

A technical question: I attached the table image by dragging. It looks tiny, can its size by changed?

Thanks, Holger, much appreciated!

This is my source for 9 Sgr: http://simbad.u-strasbg.fr/simbad/sim-b ... BAD+search. It's a Gaia parallax, so I guess it can be more or less trusted.

Ann

[neufer]

Hello Ann!

You get the distance in parsecs by dividing 1000 with the parallax in mas; and in light years by multiplying with 3.261.
But ... I looked up the parallax for 9 Sagittarii on English Wikipedia: 0.49 ± 0.40 mas (2007).
I guess you have a more recent source.

This is my source for 9 Sgr: http://simbad.u-strasbg.fr/simbad/sim-b ... BAD+search.
It's a Gaia parallax, so I guess it can be more or less trusted.

At magnitude 5.97, 9 Sgr is near the official 5.7 brightness limit for Gaia...
and lying in the middle of a bright nebula certainly doesn't help the situation.

I would trust the 2007 Hipparcos/Wikipedia parallax of 0.49 ± 0.40 mas more
especially after the two independent 1,790 pc (= 0.56 mas) determinations:

• https://arxiv.org/abs/1205.3314
  https://arxiv.org/abs/1507.06786

The distances to 9 Sgr, M8, and NGC 6530 are uncertain, but generally estimated to be between 1,200 and 1,800 parsecs. Recent studies derive a distance around 1,250 pc for the M8 region. Erosion of the front of the molecular cloud apparently caused by 9 Sgr suggests that it lies in front of the cloud, but studies of 9 Sgr as a binary star give a distance of 1,790 pc.

[Chris Peterson]

Hello Ann!

You get the distance in parsecs by dividing 1000 with the parallax in mas; and in light years by multiplying with 3.261.
But ... I looked up the parallax for 9 Sagittarii on English Wikipedia: 0.49 ± 0.40 mas (2007).
I guess you have a more recent source.

This is my source for 9 Sgr: http://simbad.u-strasbg.fr/simbad/sim-b ... BAD+search.
It's a Gaia parallax, so I guess it can be more or less trusted.

At magnitude 5.97, 9 Sgr is near the official 5.7 brightness limit for Gaia...
and lying in the middle of a bright nebula certainly doesn't help the situation.

I would trust the 2007 Hipparcos/Wikipedia parallax of 0.49 ± 0.40 mas more
especially after the two independent 1,790 pc (= 0.56 mas) determinations:

• https://arxiv.org/abs/1205.3314
  https://arxiv.org/abs/1507.06786

The distances to 9 Sgr, M8, and NGC 6530 are uncertain, but generally estimated to be between 1,200 and 1,800 parsecs. Recent studies derive a distance around 1,250 pc for the M8 region. Erosion of the front of the molecular cloud apparently caused by 9 Sgr suggests that it lies in front of the cloud, but studies of 9 Sgr as a binary star give a distance of 1,790 pc.

I would certainly not look at Hipparcos data for this. The Gaia EDR3 data and processing pipeline is vastly superior, and certainly the data we should be looking at for 9 Sgr: 0.800 ± 0.073 mas (1250 pc). (The DR2 data was 0.851 ± 0.095 mas.)

[neufer]

Click to play embedded YouTube video.

[VictorBorun]

So, you math people out there, I challenge you! Will you figure out the distances to these stars for me?

I applied the figures for facebook 3d AI, but it turned out no good
https://www.facebook.com/victor.borun/p ... 8519499897

[neufer]

So, you math people out there, I challenge you!
Will you figure out the distances to these stars for me?

You get the distance in parsecs by dividing 1000 with the parallax in mas; and in light years by multiplying with 3.261.

I applied the figures for facebook 3d AI, but it turned out no good
https://www.facebook.com/victor.borun/p ... 8519499897