'The Tail of a Wonderful Star' (APOD 17 Aug 2007)

[Seraphim Larsen]

Hello, I am a long-time fan of APOD, but am new to this forum. Today's APOD photo is really interesting -- "The Tail of a Wonderful Star" (http://antwrp.gsfc.nasa.gov/apod/ap070817.html). For me, it raised all kinds of questions.

(1) I was wondering about the mechanism that causes the variability. On Wikipedia (http://en.wikipedia.org/wiki/Mira_variable), it says the variability is caused by repeated contraction and expansion of the star. Mira's variability, from nearly invisible to magnitude 3.5, seems to be extraordinary. Is there any data to suggest the smallest and largest sizes that Mira attains? What causes the expansion and contraction? Perhaps when Mira gets too large, its thermonuclear engine slows down, which reduces the outward pressure? And so Mira starts to shrink, but this causes the thermonuclear engine to heat up again, which causes the external pressure to resume? Why would this happen with some red giants but not others?

(2) Regarding the newly-discovered trail of material, what would be the mechanism to cause the "outer layers of material blowing off into interstellar space"? What's doing the blowing?

(3) The estimated speed of the star as it moves through interstellar space is given as 130 km/sec. How are speeds like that calculated? Is that speed unusual for stars within our galaxy? What might be causing it?



Thanks again for the always-thought-provoking and beautiful photos!

[Case]

Another two questions:
For an object to create a shockwave as imaged, I would expect that object to be in a different position (yellow), then where Mira is (green). So why is the 'wave' not symmetrical around Mira?


The tail seems to be curve shaped. Does that mean Mira is traveling on that curve, instead of a straight line? If so, I would expect Mira to have a companion star or something to explain its path. If not, then maybe its shape is because of the emitting streams rotating in altering directions? Does that make sense?

[Kim]

Given that the tail started forming at it's current start position (hypothetical) I calculate that the star has been leaving this trail for around 30,000 yrs based on it's current speed.

Anyone confirm or correct this?

Mira is a known double star and is also part of a Binary system

[starnut]

Given that the tail started forming at it's current start position (hypothetical) I calculate that the star has been leaving this trail for around 30,000 yrs based on it's current speed.

Anyone confirm or correct this?

Mira is a known double star and is also part of a Binary system

If you click on the highlighted term "The discovery" in the explanation, that will take you to the GALEX web page on Mira and then click at the bottom of that page to get the full text of the press release, you will be able to confirm your calculation.

Double star and binary mean the same thing.

[starnut]

(1) I was wondering about the mechanism that causes the variability. On Wikipedia (http://en.wikipedia.org/wiki/Mira_variable), it says the variability is caused by repeated contraction and expansion of the star. Mira's variability, from nearly invisible to magnitude 3.5, seems to be extraordinary. Is there any data to suggest the smallest and largest sizes that Mira attains? What causes the expansion and contraction? Perhaps when Mira gets too large, its thermonuclear engine slows down, which reduces the outward pressure? And so Mira starts to shrink, but this causes the thermonuclear engine to heat up again, which causes the external pressure to resume? Why would this happen with some red giants but not others?

(2) Regarding the newly-discovered trail of material, what would be the mechanism to cause the "outer layers of material blowing off into interstellar space"? What's doing the blowing?

(3) The estimated speed of the star as it moves through interstellar space is given as 130 km/sec. How are speeds like that calculated? Is that speed unusual for stars within our galaxy? What might be causing it?

(1) I am not sure what causes the variability in the star's brightness, but its core continues to "burn" helium at the same temperature, never shutting down. It could be due to the core being variable in its energy output, making the outer layers swell up with more heat, making it dimmer, and shrink with less heat, making it brighter. It could be due to its white dwarf companion drawing matter off in different quantities as the orbit changes. The other red giants probably do have variable brightness at some stage of their relatively short lifespans. Near the end of a star's red giant phase, the star is going through convulsions that throw off the entire outer layers. This leaves the core exposed as a white dwarf with surface temperature of at least 100,00 degrees. The ultraviolent light from the hot surface then makes the casted off gas glow in gorgeous colors. This is what caused the planetary nebulae to be formed!

(2) When the star's envelope swells up to the red giant stage, the star's surface would be much farther from the core, so its surface gravity would be much less, making it easy for the matter at the surface to be blown off.

(3) I would guess that they measured the star's motion against the background stars and, given its distance from Earth, calculated its velocity.

[Kim]

Starnut, thanks for the path to the Galex site I never spotted that.

Double star and Binary though are not quite the same thing, many listed doubles are visual only (Albireo in Cygnus for example) and have no physical association, whereas a binary system is a pair (or more) of stars in gravitational contact.

Thanks for your comment on the art!!

[starnut]

Here is another APOD picture of Mira, taken by the Chandra x-ray telescope.

http://antwrp.gsfc.nasa.gov/apod/ap060722.html

Clicking on one of the highlighted links will give you more information about Mira.

[bystander]

(1) I was wondering about the mechanism that causes the variability. On Wikipedia (http://en.wikipedia.org/wiki/Mira_variable), it says the variability is caused by repeated contraction and expansion of the star. Mira's variability, from nearly invisible to magnitude 3.5, seems to be extraordinary...

According to the wiki-article you cited, Mira-variables are late stage red giants, in the process of becoming white dwarfs. The expansion/contraction with the accompanying changes in temperature and luminosity are the main sources of variability. Occlusion by the companion white dwarf may also affect the apparent luminosity. The lumionosity of the white dwarf may also be affected by its accretion rate. It seems to me that the expansion and contraction cycle of the red giant would affect the amount of material being accreted by the white dwarf.

Given that the tail started forming at it's current start position (hypothetical) I calculate that the star has been leaving this trail for around 30,000 yrs based on it's current speed.

According to Wikipedia, http://en.wikipedia.org/wiki/Mira, 30,000 years is spot-on.

[neufer]

Explanation: To seventeenth century astronomers, Omicron Ceti or Mira was known as a wonderful star, a star whose brightness could change dramatically in the course of about 11 months. Mira is now seen as the archetype of an entire class of long-period variable stars. Surprisingly, modern astronomers have only recently discovered another striking characteristic of Mira -- an enormous comet-like tail nearly 13 light-years long. The discovery was made using ultraviolet image data from the Galaxy Evolution Explorer (GALEX) satellite. Billions of years ago Mira was likely similar to our Sun, but has now become a swollen red giant star, its outer layers of material blowing off into interstellar space. Fluorescing in ultraviolet light, the cast off material trails behind the giant star as it plows through the surrounding interstellar medium at 130 kilometers per second. The amount of material in Mira's tail is estimated to be equivalent to 3,000 times the mass of planet Earth. About 400 light-years away toward the constellation Cetus, Mira is presently too faint to be seen by the unaided eye, but will become visible again in mid-November.

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. Hamlet > Act III, scene II

HAMLET: Do you see yonder cloud that's almost in shape of a camel?

LORD POLONIUS: By the mass, and 'tis like a camel, indeed.

HAMLET: Methinks it is like a weasel.

LORD POLONIUS: It is backed like a weasel.

HAMLET: Or like a whale?

LORD POLONIUS: Very like a whale.
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<<The variability of Mira was recorded by the astronomer David Fabricius beginning on August 3, 1596. Observing the planet Mercury, he needed a reference star for comparing positions and picked a previously unremarked third-magnitude star nearby. By August 21, however, it had increased in brightness by one magnitude, then by October had faded from view. Fabricius assumed it was a nova, but then saw it again on February 16, 1609.>>

A pair of star-crossed lovers take their life,

A stare-crossed glover:

"William Shakespeare was the son of John Shakespeare, a successful glover."

[neufer]

<<When a [low to intermediate mass] star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star's luminosity increases greatly, and it becomes a red giant, following a track leading into the upper-right hand corner of the HR diagram. The star may become as large as one astronomical unit. After the helium shell runs out of fuel, the [T]hermally [P]ulsing [A]symptotic [G]iant ranch (TP-AGB) phase starts. Now the star derives its energy from fusion of hydrogen in a thin shell, inside of which lies the now inactive helium shell. However, over periods of 10,000 to 100,000 years, the helium shell switches on again, and the hydrogen shell switches off, a process known as a helium shell flash or thermal pulse. Due to these pulses, which only last a few thousand years, material from the core region is mixed into the outer layers, changing its composition, a process referred to as dredge-up. Because of this dredge-up, AGB stars may show S-process elements in their spectra. Subsequent dredge-ups can lead to the formation of Carbon stars.

[A]symptotic [G]iant ranch stars are typically long period variables, and suffer large mass loss in the form of a stellar wind. A star may lose 50 to 70% of its mass during the AGB phase.

The extensive mass loss of AGB stars means that they are surrounded by an extended circumstellar envelope (CSE). Given a mean AGB lifetime of one Myr and an outer velocity of 10 km/s, its maximum radius can be estimated to be roughly 30 light years. This is a maximum value since the wind material will start to mix with the interstellar medium at very large radii. The outer layers of the CSE show chemically interesting processes, and due to size and lower optical depth are easier to observe.

After these stars have lost nearly all of their envelopes, and only the core regions remain, they evolve further into short lived preplanetary nebulae. The final fate of the AGB envelopes are represented by planetary nebulae (PNe).>>

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<<Mira A is currently an Asymptotic Giant Branch (AGB) star, in the thermally pulsing AGB phase. Each pulse lasts a decade or more, and an amount of time on the order of 10,000 years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger. This is also causing dynamic instability in Mira, resulting in dramatic changes in luminosity and size over shorter, irregular time periods.

The overall shape of Mira A has been observed to change, exhibiting pronounced departures from symmetry. These appear to be caused by bright spots on the surface that evolve their shape on time scales of 3–14 months. Observations of Mira A in the ultraviolet band by the Hubble Space Telescope have shown a plume-like feature pointing toward the companion star.

Mira A is a well-known example of a category of variable stars known as Mira variables. It—and the other ca 6000-7000 known stars of this class—are all red giants whose surfaces oscillate in such a way as to increase and decrease in brightness over periods ranging from about 80 to more than 1000 days.

In the particular case of Mira, its increases in brightness take it up to about magnitude 3.5 on average, placing it among the brighter stars in the Cetus constellation. Individual cycles vary too; well-attested maxima go as high as magnitude 2.0 in brightness and as low as 4.9, a range almost 15 times in brightness, and there are historical suggestions that the real spread may be three times this or more. Minima range much less, and have historically been between 8.6 and 10.1, a factor of four times in luminosity. The total swing in brightness from absolute maximum to absolute minimum (two events which did not occur on the same cycle) is 1700 times. Interestingly, since Mira emits the vast majority of its radiation in the infrared, its variability in that band is only about two magnitudes. The shape of its light curve is of an increase over about 100 days, and a return twice as long.
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• . The Tempest – Act 1, Scene 2
  .
  MIRA-nda: The sky, it seems, would pour down stinking pitch,
  . But that the sea, mounting to the welkin's cheek,
  . Dashes the fire out.

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In 1638 Johann Holwarda determined a period of the star's reappearances, eleven months; he is often credited with the discovery of Mira's variability. Johannes Hevelius was observing it at the same time and named it "Mira" (meaning "wonderful, astonishing") in 1662's Historiola Mirae Stellae, for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less than one day off the modern value of 332 days (and perfectly forgivable, as Mira is known to vary slightly in period, and may even be slowly changing over time).>>

[ILD]

A question somewhat related to this subject, in a general way, is that of where precisely does solar nuclear fusion occur? Is it homogeneous throughout the nuclear core, or does it occur in a "shell" between the core and the radiant zone. If homogeous, are the pressure, temperature and other factors throughout the core within the required range for fusion to occur at a steady state? If in a shell, how does the fusion zone develop from initiation of fusion to form a shell? Can this question be resolved by models of nuclear processes?