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