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RJ Emery
2007-Aug-17, 01:26 AM
Recent images of the red supergiant Mira detected for the first time a 13 ly tail fluorescing in the ultraviolet part of the spectrum. Thinking back, I was wondering to myself how many millions of years it took for this tail to form and what Mira as a star may have been when it first began shedding star material in this fashion. Any thoughts?

Hornblower
2007-Aug-17, 02:50 AM
The Washington Post had a nice article this morning.

http://www.washingtonpost.com/wp-dyn/content/article/2007/08/15/AR2007081501422.html

Mira spent several billion years as a main sequence hydrogen burner similar to our Sun, and then some hundreds of millions of years in a series of bloated stages before reaching the state in which it started forming the tail. That appears to be upwards of 30,000 years ago, just a twinkling in proportion to its entire lifetime.

01101001
2007-Aug-17, 03:03 AM
Any thoughts?

I wouldn't be so bold as to label it an actual thought, but a back of the Google calculation yields...

(13 lightyears) / (80 (miles / second)) = 30 270.8895 years

(0.8895 years = 324.882936 days!)

RJ Emery
2007-Aug-17, 06:56 AM
The Washington Post had a nice article this morning.

http://www.washingtonpost.com/wp-dyn/content/article/2007/08/15/AR2007081501422.html

Mira spent several billion years as a main sequence hydrogen burner similar to our Sun, and then some hundreds of millions of years in a series of bloated stages before reaching the state in which it started forming the tail. That appears to be upwards of 30,000 years ago, just a twinkling in proportion to its entire lifetime.

Yes, 30,000 years is a blink of the eye cosmically. I do find one statement in the Washington Post article to be difficult to accept:


Because of its great size, the gravitational force keeping it together is weak at the edges, allowing the winds created by the fast-moving star to pull matter off and send it back into the tail.

Winds? All non-exotic stars generate winds of their own material. As Mira moves through interstellar space, I can understand how material so emanated is left behind, leaving a tail. However, the article states Mira's winds themselves are responsible for presumably even more matter being stripped from the star, and that I find difficult to comprehend.

Hornblower
2007-Aug-17, 12:14 PM
Yes, 30,000 years is a blink of the eye cosmically. I do find one statement in the Washington Post article to be difficult to accept:



Winds? All non-exotic stars generate winds of their own material. As Mira moves through interstellar space, I can understand how material so emanated is left behind, leaving a tail. However, the article states Mira's winds themselves are responsible for presumably even more matter being stripped from the star, and that I find difficult to comprehend.
I see sloppy use of "great size" which can confuse a novice by implying "great gravity" at the photosphere, in apparent contradiction to the statement that the gravity is weak at that radius.

The mass is estimated to be about one solar mass, but the superheated evolved core is putting out several thousand times as much energy per second, and in the process is puffing the envelope up to some 400 times the radius. At that great radius the gravity is reduced to about 1/160,000 of the solar photosphere value, so the outlying gas escapes more readily.

RJ Emery
2007-Aug-17, 12:27 PM
The mass is estimated to be about one solar mass, but the superheated evolved core is putting out several thousand times as much energy per second, and in the process is puffing the envelope up to some 400 times the radius. At that great radius the gravity is reduced to about 1/160,000 of the solar photosphere value, so the outlying gas escapes more readily.
Ostensibly then, all red supergiants should be emanating gases about the same as Mira. Since their velocities are substantially less than that of Mira, all that develops is a shroud or thicker photosphere surrounding the star. By contrast, Mira should have a relatively thin photosphere.

antoniseb
2007-Aug-17, 01:33 PM
all that develops is a shroud or thicker photosphere surrounding the star. By contrast, Mira should have a relatively thin photosphere.

I think you may be confused about what the photosphere is. The term 'shroud' kind of works here, but it is from material ejected from the star, whereas the photosphere is material still in the star.

RJ Emery
2007-Aug-17, 03:56 PM
I think you may be confused about what the photosphere is. The term 'shroud' kind of works here, but it is from material ejected from the star, whereas the photosphere is material still in the star.

You may well be correct. If a star has a surface, it is my understanding that the subphotosphere lies above it, followed by the photosphere, then the chromosphere, followed by the corona, all of which comprise the atmosphere. Feel free to correct or amplify if that is not the case.

The photosphere is defined by Gray (The Observation and Analysis of Stellar Photospheres, 2/e, 1992) as that major portion of the atmosphere where the visible stellar spectrum originates (p.1). His exact words are:


For an observer of stellar atmospheres, one concept is very important: the major portion of the visible stellar spectrum originates in the region marked 'photosphere.'
I do not know if that is to be considered within the star or not. I rather suspect it is not. To my understanding, within the star would imply below the subphotosphere and refer to the star proper.

In the case of other red supergiants, I do not know how the atmospheric structure would differ, and in the case of Mira, where the gas the comprises the tail would start given that structure. I somehow think outgassing by supergiants would somehow modify the structure we think of in reference to normal stars.

TaylorS
2007-Aug-19, 12:05 AM
Mira is an Asymptotic red giant (the final part of the giant phase), correct? Perhaps the transition to the white dwarf phase is starting and this tail is the result of Mira starting the process of shedding it's outer layers.

RJ Emery
2007-Aug-19, 12:26 AM
Mira is an Asymptotic red giant (the final part of the giant phase), correct? Perhaps the transition to the white dwarf phase is starting and this tail is the result of Mira starting the process of shedding it's outer layers.
Perhaps. Other red supergiants have velocities but shed their extra mass in a spherical fashion, giving rise to planetary nebulae.

Urbane Guerrilla
2007-Aug-20, 08:53 AM
Though I'll bet a bunch of grad students and undergrads are now looking for trails from any high proper motion red giants they can find. They ought to be there. And how about clumpy trails from long period variables? Cepheids?

publiusr
2007-Aug-24, 08:01 PM
If you were to steer a star--would it look like this?

Urbane Guerrilla
2007-Aug-25, 05:43 AM
I don't know what's causing the arc shape -- galactic magnetic fields? Interstellar medium moving rather like a solar wind? Some imaging artifact, such as is familiar to every nearsighted person seeing straight lines getting refracted near the edge of his glasses lenses?

mfumbesi
2007-Aug-27, 07:47 AM
I have thoughts on Mira which might be off-topic.
The "tail" is 13 lyrs:
->Stars "behind" Mira will plough through the material left behind by Mira, if you can Imagine our solar system being one of those stars I think that will be the end for us.
->When the same process happens to our sun in the far future (I know life would have long ceased on earth), the extra material be swept by the surviving planets, I wonder if that wont heat and melt earth. Jupiter ( and the other gas giants) will put on considerable amount of mass.
->If Mira was say 50 lyrs, this would be one spectacular show for us.
->Any closer we would be in trouble, this led me to think that you don't need a supernova in the neighborhood to cause serious disturbances (possible extinction of life on neighboring "solar systems").
->The BA should add a chapter on his new book about a similar event happening in our neighborhood.

Any thoughts?

StupendousMan
2007-Aug-27, 01:42 PM
I have thoughts on Mira which might be off-topic.
The "tail" is 13 lyrs:
->Stars "behind" Mira will plough through the material left behind by Mira, if you can Imagine our solar system being one of those stars I think that will be the end for us.
->When the same process happens to our sun in the far future (I know life would have long ceased on earth), the extra material be swept by the surviving planets, I wonder if that wont heat and melt earth. Jupiter ( and the other gas giants) will put on considerable amount of mass.
->If Mira was say 50 lyrs, this would be one spectacular show for us.
->Any closer we would be in trouble, this led me to think that you don't need a supernova in the neighborhood to cause serious disturbances (possible extinction of life on neighboring "solar systems").


Just because it looks pretty doesn't mean it's dangerous. I suggest you calculate the density of material in the tail. You might assume that -- to within an order of magnitude -- a total of 0.5 solar masses of gas has been spread out into a rectangular box which is 13 light years long and 0.5 light years wide in cross section. Assume that the gas is hydrogen. How many atoms are there per cubic centimeter, or per cubic meter?

Now assign a temperature to the gas ... let's say 10,000 Kelvin. Compute the energy per cubic meter. Compare it to the energy in a cubic meter of Earth's atmosphere at sea level with a temperature of 20 Celsius = 293 Kelvin -- a nice summer's day.

Figure out the cross section area of the Earth. How many cubic meters of the tail will the Earth sweep up? What is the total energy of the gas within that volume of the tail?

Peter Wilson
2007-Aug-27, 05:50 PM
I plugged in the numbers, and the results are not pretty: for passengers on Earth, hitting Mira's tail would be like the Titanic running full-throttle into a smoke-ring :shifty:

George
2007-Aug-28, 02:12 AM
You may well be correct. If a star has a surface, it is my understanding that the subphotosphere lies above it, followed by the photosphere, then the chromosphere, followed by the corona, all of which comprise the atmosphere. Feel free to correct or amplify if that is not the case.Ok, here is a nit. I think you meant the "subphotosphere lies below it". I have not seen much on this region but it seems to be important to helioseismology, at least. Also, don't forget the large heliosphere. :)


The photosphere is defined by Gray (The Observation and Analysis of Stellar Photospheres, 2/e, 1992) as that major portion of the atmosphere where the visible stellar spectrum originates (p.1). Of course, that's what the average camera photo will reveal. ;)


I do not know if that is to be considered within the star or not. I rather suspect it is not. The photosphere is quite distinct and is considered a star's surface, I would think.

Doodler
2007-Aug-28, 02:19 PM
Yes, 30,000 years is a blink of the eye cosmically. I do find one statement in the Washington Post article to be difficult to accept:



Winds? All non-exotic stars generate winds of their own material. As Mira moves through interstellar space, I can understand how material so emanated is left behind, leaving a tail. However, the article states Mira's winds themselves are responsible for presumably even more matter being stripped from the star, and that I find difficult to comprehend.

If Mira is ending the hydrogen burning phase of its life, the higher energy emissions of helium fusion is pushing the star towards what would normally be its first red giant phase (which bloats the star to impressive size as the outer layers are "fluffed" by the more intense core emissions). This will result in far more material being ejected than would be expected from everyday coronal mass ejections.

Given that its moving at a pretty impressive clip, the bloated outer layers are actually being shedded to form the tail.

Amber Robot
2007-Aug-28, 02:46 PM
If Mira is ending the hydrogen burning phase of its life, the higher energy emissions of helium fusion is pushing the star towards what would normally be its first red giant phase (which bloats the star to impressive size as the outer layers are "fluffed" by the more intense core emissions). This will result in far more material being ejected than would be expected from everyday coronal mass ejections.

From the Nature paper reporting this discovery:


Mira is a binary system with an orbital period of ~500 years. The primary star, Mira A, is the luminous, mass-shedding, evolved star that exhibits long-period, semi-regular variability. These properties arise because Mira A has exhausted teh supply of hydrogen and helium in its core and at present is powered by helium and hydrogen fusion in a shell surrounding a carbon-oxygen core. These physical conditions typify what are known as asymptotic giant branch (AGB) stars.


Given that its moving at a pretty impressive clip, the bloated outer layers are actually being shedded to form the tail.

Also from the Nature paper:


The appearance of a bow shock is consistent with Mira's large space velocity of 130 km/s, which we calculate from the proper motion and the radial velocity...