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...

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• 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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TAGS: Astronomy, Mira, Omicron Ceti