Saturday, August 25, 2007

Mira, the Marvellous Star

You have probably seen already here, or here, this wonderful picture of a comet-like tail at a star:



It shows a trail of dust and gas the "asymptotic giant branch" star Mira has left behind, while speeding through the interstellar gas of our galaxy at 160 km per second. This tail extends over nearly two degrees in the sky, which means, given the distance to Mira, that it is more than 12 light years long! Unfortunately, one can not see it in optical telescopes: It is visible only in the ultraviolet. That's why no one had spotted it before this photograph was shot by the Galex (Galaxy Evolution Explorer) space telescope.

The tail of Mira can be seen only in ultraviolet light (top). In usual visible light (bottom), there seems to be nothing special around Mira (Source: Galex team, Caltech)

Mira is a star with a mass a little larger than our Sun's, but with a radius nearly as big as the orbit of Mars. Its surface temperature is a mere 2200 K, but due to its size, its total output of radiation is 8400 as big as that of the Sun.

Mira is in the late phase of its life as a star: it has a core of carbon and oxygen, which is surrounded by two layers where helium and the remains of hydrogen are burned. It is pulsating slowly, with a period of 332 days, and constantly blowing away quite big amounts of gas and dust - one tenth of the Mass of Earth per year - into its surroundings. This material is colliding with the interstellar medium. Du to the high proper speed of Mira, it is stopped in a bow shock hat is clearly visible on the Galex photos, and left behind as the tail. Julianne at Cosmic Variance has discussed in much more detail the fascinating astrophysics of Mira and shock phenomena in astronomy in general.

But even though Mira's tail doesn't reveal itself to the eyes of mundane hobby astronomers as me, the star is an interesting object to observe even with the naked eye: In the course of its 332 period, its brightness diminishes so much that it completely drops out of sight! That's in fact how Mira got its name.

In August 1596, Frisian cleric and spare-time astronomer David Fabricius was observing planet Mercury, when he spotted a star in the constellation of Cetus, the Whale, that he had never seen before and could not find in the catalogues and maps of stars where he tried to look it up. He knew about novae, stars that lighten up for some time and fade away again, such as Tycho's Nova of 1572, and thought that he had discovered one - and indeed, this new star increased its brightness first, but by October, it had become so dim that Fabricius lost it from view.

Mira, or Omicron Ceti, in Bayers 1603 stellar chart Uranometria: In this detail, it's the star right at the centre. (Source: Linda Hall Library of Science, Engineering, & Technology

But then, this funny star reappeared: Fabricius spotted it again in 1609, and independent of him, astronomer Johann Bayer had registered it in his map of stars, the Uranometria, and denoted it as "Omicron Ceti" according to the labelling system he had devised for his map - a name that is still in use today.

A systematic analysis of the different available observations by astronomer Holwarda in 1638 revealed that this star, Omicron Ceti, was a variable star, which changed its brightness on a regular basis with a period of 332 days. This was such strange a phenomenon that Hevelius, one of the most influential astronomers of the time, dubbed the star simply Mira stella, the marvellous star.

The change in visual brightness of Mira is dramatic, indeed. The diagram below, a collection of observations provided by the American Association of Variable Star Observers (AAVSO), shows the magnitude of Mira over the last 400 days: At the last maximum, in February, the magnitude peaked around 2...3, while the right now, Mira is close to its minimum, at mag 9. That's a difference by more than a factor hundred! Remember that the threshold for the visibility of stars by the naked eye is around mag 5...6, so right now, one needs a small telescope to spot Mira!

The light curve of Mira over the last 400 days. The approximate threshold for visibility by the naked eye is marked in red (Source: AAVSO)

The physical reason for this shift in brightness of Mira is quite interesting: On the one hand, the star is pulsating, and whenever it expands, its cools, and the maximum of its black-body-like emission spectrum shifts away from the visible - at an average surface temperature of 2200 K, it's in the infrared anyway. This means that the largest chunk of radiation is emitted in the infrared, and not in the visible part of the spectrum. Thus, in the infrared, changes in brightness would be much less dramatic.

But then, there is a second, funny effect, which enhances the change in brightness: If Mira cools down in the expanding phase of its pulsation, titanium monoxide molecules condense in its vast atmosphere, and these molecules absorb nearly completely the visible light from the inner layers of the star. I wonder whether there is a relation to the bright white colour of titanium dioxide...

So far, I have never tried to follow the appearance and the fading of Mira in the sky - but this winter, I will try to do so - the next maximum is expected for the first half of January 2008. Cetus the Whale is not such bright a constellation. However, located near the celestial equator, west of Orion, and in the southeast of the square of Pegasus, it is quite well visible in fall and winter from the Northern hemisphere. There are plenty of sky charts that help to locate Mira - but in general, wikisky.org is a good address to start:

Mira in Cetus west of Orion (Source: wikisky.org)

At wikisky, they have even already integrated the Galex photo of the tail of Mira, so that one can see how it actually fits into the sky:

The Galex UV photograph of the tail of Mira at wikisky.org

But we have to keep in mind that unfortunately, even at Mira's maximal visual brightness, we can not see the spectacular tail...




  • The original Galex results have been published last week in Christopher D. Martin et al.: A turbulent wake as a tracer of 30,000 years of Mira’s mass loss history, Nature 448 (2007) 780-783; DOI: 10.1038/nature06003 (subscription required)
  • The paper about the dimming effect of titanium monoxide forming in Mira's atmosphere is M. J. Reid and J. E. Goldston: How Mira Variables Change Visual Light by a Thousandfold, The Astrophysical Journal 568 (2002) 931-938 (available online for free)
  • A comprehensive history of Mira is Dorrit Hoffleit: History of Mira's Discovery, online from the AAVSO.



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13 comments:

dark-matter said...

Mira is interesting. How about the Huge Hole in the Universe discovery?

http://www.space.com/scienceastronomy/070823_huge_hole.html

This hole, discovered from CMB data analysis, does not even contain dark matter. Which, unlike Mira, may have new implications in fundamental physics. Just wondering why you pick Mira and not the Big Hole.

Francis Caestecker said...

Oeew oeh! The hole :D. I'm looking forward to that too ;).

Anyways, nice and educating article! *thumps up*

Bee said...

Thanks for that marvellous post :-) Mira is a beautiful name. Now look how they name things nowadays. Wimpzilla? Sfermion? Unparticles? The only thing that comes into my mind with a somewhat poetical appeal is 'rainbow gravity', the poety of which however seems to be borrowed from Gravity's Rainbow (and for some theories on where this title comes from, see here)

stefan said...

Hi dark-matter, francis:


ah, the big black void... thank's for the hint. I've so far only found the original press release, and lots of paraphrasing that text in other reports - the paper is arXiv:0704.0908v2.

Just wondering why you pick Mira and not the Big Hole.

I usually don't have time to write longer posts like this one within one day or two - this is usually not a "latest news" blog.

Moreover, you may have noticed that the point (at least, that was my intention) of the post is that Mira is a star you can see appear in the sky every 330 days or so with the naked eye, without any technological help. I knew a bit about Mira before, and always had wanted to see it show up in the night sky ... this news about the tail was a good motivation to write about this, and indeed, I plan to have an eye at Mira this late fall ;-).

The situation with the cold spot is much more complicated, observations so far seem to be (please correct me if I am wrong) with microwaves and radio only, and you need quite an involved statistical analysis before you can guess that a big void may be the best explanation of the data.

And I guess comments should be best by people who know more about the integrated Sachs-Wolf effect than just the name ;-)


Best, stefan

stefan said...

Dear Bee,

Mira is a beautiful name. Now look how they name things nowadays. Wimpzilla? Sfermion? Unparticles?

Mira is a beautiful name indeed! And "rainbow gravity", that's poetic.

Maybe people should know more Latin. Or read more obscure books such as Finnegan's Wake ;-)

Best, stefan

dark-matter said...

Stefan,
Thanks. Do enjoy reading Mira. Suspect this Huge Hole thing may become more important after further analysis at which point you might wish to say something. The CMB is far more revealing than once thought.

Anonymous said...

Maximum is January 2008 or thereabouts, eh? Funnily enough I've been doing amateur astronomy for, say, 20 years (boy do I feel old!) and I've never yet seen Mira. Maybe I'll check it out next maximum; thanks!

changcho

Arun said...

Mira/Meera is a common name for girls in India; likely the most famous instance. Also.


Anyway, Stefan, this Mira made me wish my childhood dream, that I could travel between the stars....

Bee said...

Hi Stefan,

Well, I've always found the name 'quark' very silly. I guess neither Gell-Mann nor Joyce knew it's the German word for 'curd cheese', colloquially used for 'nonsense'. I.e. "So ein Quark" means "This is nonsense." Best,

B.

stefan said...

Hi changcho,

Funnily enough I've been doing amateur astronomy for, say, 20 years (boy do I feel old!) and I've never yet seen Mira.

It's exactly the same with me! Let's hope that we will get around to seeing Mira this time ;-)

Best, stefan

Doug said...

Hi stefan,

Could there be any relation of Mira to Herbig-Haro [HH] Objects?

Mira old, HH new?

http://en.wikipedia.org/wiki/Herbig-Haro_object

stefan said...

Hi Doug,

thank you for pointing out the Herbig-Haro objects to me - I didn't know about them before!

From what I learn from the Wikipedia entry (be aware I am not an astronomer - restrictions apply), both Mira's tail and the HH objects are caused by shock phenomena, which may explain the striking similarity between the Mira photo and the HST photo of HH47 on the Wikipedia page. But the mechanism that are responsible for occurence of these shocks seem to be quite different:

Mira is an old star which is pushing away the outer layers of its atmosphere equally in all directions. For other, similar old stars, such processes may result in the formation of a beautiful "planetary nebula". But since Mira has a quite big velocity with respect to the interstallar gas of the Galaxy, the "head wind" by that gas blows away these spherical shells which would otherwise form the planetary nebula, and causes the tail.

The HH objects, in contrast, are young stars, and they blow away parts of the gas they are forming from along their axis of rotation. In these jets, this gas is accelerated to quite a high speed with respect to the surrounding medium (the young star is at rrest with respect to this medium), and this causes the characteristic lobes of the HH objects.

In case an astronomer who knows more about this is reading the comments, please correct me if I am wrong...

Best, stefan

Paul Sutherland said...

A shame that some of you missed out on seeing Mira earlier this year. It was the brightest maximum I had ever seen and it was odd to see her shining so brightly in London evening twilight. Unfortunately, I think there is a "tradition" for faint maxima to follow bright ones. We will have to wait and see!