Explanation: Do you recognize this constellation? Although it is one of the most recognizable star groupings on the sky, this is a more full Orion than you can see -- an Orion only revealed with long exposure digital camera imaging and post-processing. Here the cool red giantBetelgeuse takes on a strong orange tint as the brightest star on the upper left. Orion's hot blue stars are numerous, with supergiantRigel balancing Betelgeuse on the lower right, and Bellatrix at the upper right. Lined up in Orion's belt are three stars all about 1,500 light-years away, born from the constellation's well-studied interstellar clouds. Just below Orion's belt is a bright but fuzzy patch that might also look familiar -- the stellar nursery known as Orion's Nebula. Finally, just barely visible to the unaided eye but quite striking here is Barnard's Loop -- a huge gaseous emission nebula surrounding Orion's Belt and Nebula discovered over 100 years ago by the pioneering Orion photographer E. E. Barnard.
1) Why it's called a "loop" when all we can see (at least prominently and obviously) is a C shaped arc ?
I did quick Google search and couldn't see an answer in a few clicks
2) How do they measure distances to such objects and why it has large margin of error (~500 to ~1400 light years) ?
I had to aggressively shrink the APOD to make it fit on my screen, so that I could grab a print screen and fill in a number of additional stars and nebulas, so if you are interested in my annotated image, it's a very good idea to enlarge it.
Let's look at some sights in the Orion constellation (or, in the case of the Witch Head Nebula, in neighboring Eridanus).
And finally (phew!) NGC 1788! Did you ever see such a funny-looking cosmic creature? I can't help thinking of Rudolph, the red-nosed Reindeer. But NGC 1788 is a white-nosed... something.
Wait, I'm not done! I just have to show the amazing Pickett's Bell around Betelgeuse, which also seems to be connected to both the Lambda Orionis Nebula and the Rosette Nebula!
Pickett's Bell around Betelgeuse, with a "string"
connecting it to the Rosette Nebula at top right.
Lambda Orionis is near bottom, and it seems to "hang"
from Pickett's Bell. Barnard's Loop is at left.
Credit: Miguel Claro.
Doesn't it look amazing???
Okay, edit... My third edit...
I've got to show you a brilliant closeup of NGC 2023, a large reflection nebula "at the foot of" (or the neck of?) the Horsehead Nebula:
shaileshs wrote: ↑Tue Jan 16, 2024 6:05 am
Quick question on Barnard's loop -
1) Why it's called a "loop" when all we can see (at least prominently and obviously) is a C shaped arc ?
I did quick Google search and couldn't see an answer in a few clicks
2) How do they measure distances to such objects and why it has large margin of error (~500 to ~1400 light years) ?
Thanks in advance to all who respond.
Why is Barnard's Loop called a loop? There would seem to be no reason for it. I sometimes call it Barnard's Arc myself.
I googled "Barnard's Loop wiki" to see if Wikipedia had anything to say about the popular moniker. They didn't. But I guess it's possible that early hydrogen alpha photographs of Orion made Barnard's Arc look very slightly loop-like.
Why is it hard to measure distances to objects in Orion? The short answer is that we have access to two telescopes that measure distances to objects in space by measuring their parallax. The two telescopes are Hipparcos and Gaia. Unfortunately, neither of them can really measure the distances to the bright stars in Orion!
Hipparcos uses parallax to determine distances to stars. Basically, it measures how much a star appears to move back and forth in the sky as the Earth moves from one "end" of its orbit (say, in March) to the other extreme "end" of its orbit (say, in September). How much the star seems to move during these two "measuring points" (that must always be 6 months apart) determines its distance from the Earth.
I highly, highly recommend that you watch this video:
Click to play embedded YouTube video.
The problem with Hipparcos is that it can only reliably measure the distances to stars that are closer to us than ~300 light-years. The stars of Orion are much farther away than that. Hipparcos has indeed measured distances to the stars in Orion, but the distances are not reliable. For example, according to Hipparcos, Alnilam (the middle star of Orion's Belt) is twice as far away from us as the two other Belt stars.
The other great measuring tool that is available to us is ESA's Gaia telescope. Gaia measures distances with a much greater accuracy than Hipparcos. Unfortunately, Gaia can't measure distances to stars that are too bright. All the important stars in Orion are too bright for Gaia.
There are other ways to calculate distances to stars, particularly by looking at the spectra of stars. Stars show spectral lines that are determined by the temperature of the star:
By looking at a star's spectrum, astronomers can determine the star's spectral class. (Even that is tricky at times.) They can then say how bright a star of that spectral class usually is. But most stars are actually not "standard candles". In most cases you can't say that just because a star belongs to a certain spectral class, we can know how bright it is. We can't. Consider Vega and Alioth, the brightest star in the handle of the Big Dipper. Both Vega and Alioth are spectral class A0, but the luminosity of Vega is some ~ 50 times that of the Sun, and the luminosity of Alioth is some ~100 times that of the Sun. We know that because Vega and Alioth are both so nearby that their parallaxes can be measured by Hipparcos.
It is possible that the spectra of Vega and Alioth do indeed show that Alioth must be the brighter of the two. Still, it's tricky to determine the distance to a star by looking at its spectrum, judging its intrinsic luminosity from its spectrum and calculating its distance from its supposed intrinsic luminosity.
Ann
Last edited by Ann on Tue Jan 16, 2024 4:06 pm, edited 1 time in total.
"The Orion you can almost see". I can, - in my dreams! (and thank you Ann for these wonderful additions!)
"I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion"... (cf Blade Runner, final scene). Can anyone see those in the APOD?
In reference to Ann's post about Hipparcos versus Gaia for measuring a star's parallax: both are in-space telescopes - Gaia is at the Earth-Sun L2 Lagrange Point and Hipparcos is in Earth orbit (I think) - that determine parallax by measuring the change in a star's position against background stars at two points in Earth's orbit 180° apart (i.e., 6 months apart). How does Gaia manage to be 100 times more accurate (from what I've read)?
-- "To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
johnnydeep wrote: ↑Tue Jan 16, 2024 7:08 pm
In reference to Ann's post about Hipparcos versus Gaia for measuring a star's parallax: both are in-space telescopes - Gaia is at the Earth-Sun L2 Lagrange Point and Hipparcos is in Earth orbit (I think) - that determine parallax by measuring the change in a star's position against background stars at two points in Earth's orbit 180° apart (i.e., 6 months apart). How does Gaia manage to be 100 times more accurate (from what I've read)?
There are probably some improvements created by the optics (like the larger mirrors), but I think the main difference lies in the much greater mechanical stability and pointing accuracy of Gaia, which reduces the position noise.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
johnnydeep wrote: ↑Tue Jan 16, 2024 7:08 pm
In reference to Ann's post about Hipparcos versus Gaia for measuring a star's parallax: both are in-space telescopes - Gaia is at the Earth-Sun L2 Lagrange Point and Hipparcos is in Earth orbit (I think) - that determine parallax by measuring the change in a star's position against background stars at two points in Earth's orbit 180° apart (i.e., 6 months apart). How does Gaia manage to be 100 times more accurate (from what I've read)?
There are probably some improvements created by the optics (like the larger mirrors), but I think the main difference lies in the much greater mechanical stability and pointing accuracy of Gaia, which reduces the position noise.
Ok, thanks.
-- "To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
The Orion You Can Almost See
