NGC 1309: Stepping Stone to the Universe
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Monday, Feb. 4, 2013, at 8:00 AM ET
The spectacular face-on spiral NGC 1309, a galaxy with a secret.
Click to embiggen, and, unless jaw-dropping beauty is something you
hate, you want to.
Image credit: NASA, ESA, The Hubble Heritage Team, (STScI/AURA) and A. Riess (STScI)
Image credit: NASA, ESA, The Hubble Heritage Team, (STScI/AURA) and A. Riess (STScI)
When I find a beautiful Hubble picture of a galaxy with which I’m not
familiar, the first thing I do is hit the journals and databases. How
far away is it? Why was it observed?
That first one was easy: This galaxy, called NGC 1309, is about 90
million light years away, give or take, putting it at the far side of
what we might call “nearby”. Still with Hubble’s incredible vision, NGC
1309 is displayed in amazing detail (grab the 2400 x 3200 pixel version to see for yourself).
The answer to the second question, at first, eluded me. I dove right
into the professional journals and extragalactic databases, looking to
see what was going on with this galaxy. I found nothing. No mention of a
huge black hole gobbling down matter and blasting out jets, no recent
merger with another galaxy, no anomalously bright high-energy sources in
it.
Then I laughed. I realized I didn’t actually read the synopsis of the observation on the Hubble website;
I jumped right into the deep end. But when I read the short
description, it became clear why NGC 1309 was targeted: It turns out,
while overall the galaxy itself is scientifically unremarkable, it’s
what’s in it that makes it special.
NGC 1309 has quite a few stars in it that are called Cepheid
variables. These are massive, bright stars that pulsate in brightness on
a regular schedule lasting a few days to months. The pulsation period
of a Cepheid depends on the absolute brightness of the star, how much
energy it emits. By measuring how bright the star appears over time, then, we can determine how bright the star actually
is. That in turn allows us to measure its distance (because stars dim
with distance in a relatively easy-to-determine way). Since we can see
Cepheids in distant galaxies, that allows us to measure the actual
distance to these galaxies.
That fact, all by itself, is amazing.
But NGC 1309 has another trick up its arm. On top of the Cepheids, in
2002 an exploding star was spotted in the galaxy, and it was of the
kind that can also be calibrated to calculate its distance. This kind of
supernova is used to measure the distances of galaxies that are
incredibly far away, billions of light years distant, clear to
the edge of the observable Universe. Being able to find a galaxy with
both Cepheid stars and this type of supernova is a rare chance to match
the two scales up and make sure they fit. It helps us bootstrap the size and behavior of the entire Universe.
Incredible.
That’s why NGC 1309 found itself in Hubble’s crosshairs. But to my
amusement, that’s it. It’s otherwise a completely ordinary spiral
galaxy, slightly smaller than our Milky Way, with wide-flung spiral arms
festooned with dark dust lanes. Scientifically, if I saw NGC 1309 in a
database or journal table, I wouldn’t give it a second glance.
That would be a shame, because it’s gorgeous.
The smaller galaxy near NGC 1309, showing some disturbed features.
Image credit: NASA, ESA, The Hubble Heritage Team, (STScI/AURA) and A. Riess (STScI)
Image credit: NASA, ESA, The Hubble Heritage Team, (STScI/AURA) and A. Riess (STScI)
Two other things did draw my attention in the Hubble image. First was
the bizarre, smaller galaxy to the upper left. It has almost no defined
nucleus, possesses that weird bar across it, and then has those kinked
and messed-up arms. What’s its story? It looks like it recently had a
collision with another galaxy that disrupted its structure, but I’m not
sure. After some time online poking through databases, all I could find
about it was how bright it is (17th mag, for those keeping
notes at home, or 0.004% as bright as the faintest star you can see with
your naked eye). That’s it. I suspect this Hubble image is the first
detailed picture of it ever taken, so it’s not been studied thoroughly
yet.
Second, I’ll note all the myriad background galaxies, so commonly
seen in images like this. Interestingly, they appear to tend toward the
red, which makes me think that NGC 1309 has a lot of dust in its
outskirts; blue light passing through dust gets absorbed or scattered
away, while red light passes through. It’s possible all those galaxies
are intrinsically red, but it seems unlikely. Another mystery to pass on
to someone who may one day study this lovely spiral galaxy.
I also find it somewhat funny that, above, I described this
incredible object, so huge, so fantastically beautiful, as
“unremarkable”. Perhaps as a galaxy itself it’s typical, one of many, a
big spiral among countless thousands known. Yet it possesses the keys to
our understanding of the Universe itself.
Unremarkable, indeed.
Tip o' the lens cap to APOD, which has a slightly different version of this image, reprocessed by Martin Pugh.
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