Freethought & Rationalism Archive

ps418 · 03-31-2003, 09:34 AM

In Jan 2002, a star known as V838 Monocerotis, about 200,000 light years from earth, briefly became the brightest object in our galaxy. The first explanation that comes to mind of course is a nova. But this was not a nova, because the star did not expel its outer layers. From the press release:

Quote:

The outburst of V838 Mon was somewhat similar to that of a nova, a more common stellar outburst. A typical nova is a normal star that dumps hydrogen onto a compact white-dwarf companion star. The hydrogen piles up until it spontaneously explodes by nuclear fusion � like a titanic hydrogen bomb. This exposes a searing stellar core, which has a temperature of hundreds of thousands of degrees Fahrenheit.

By contrast, however, V838 Mon did not expel its outer layers. Instead, it grew enormously in size, with its surface temperature dropping to temperatures not much hotter than a light bulb. This behavior of ballooning to an immense size, but not losing its outer layers, is very unusual and completely unlike an ordinary nova explosion.

Another very interesting thing about this event is the light echo expanding away from the star. As it expands, it illuminates the surrounding space like an "astronomical cat-scan". From the press release:

Quote:

As light from the outburst continues to reflect off the dust surrounding the star, we view continuously changing cross-sections of the dust envelope. Hubble's view is so sharp that we can do an 'astronomical cat-scan' of the space around the star," says the lead observer, astronomer Howard Bond of the Space Telescope Science Institute in Baltimore.

Images: Hubble Watches Light from Mysterious Erupting Star Reverberate Through Space

Press Release: Hubble Watches Light from Mysterious Erupting Star Reverberate Through Space

Patrick

ps418 · 03-31-2003, 09:45 AM

Forgot to add: this star is on the cover of the latest issue of Nature, and the issue contains "letter to Nature" about the phenomena:

Reflected glory: A stellar explosion like no other

Patrick

Abacus · 03-31-2003, 10:07 AM

And it was the Astronomy Picture of the Day just a few days ago.

fando · 03-31-2003, 04:13 PM

Nice! Let's see... this must be what happens to stars not massive enough to go nova but barely so. We're starting to fill in the grey areas of star evolution, or the missing links so to speak.

Tim Thompson · 03-31-2003, 07:41 PM

According to the Nature paper (An energetic stellar outburst accompanied by circumstellar light echoes, Howard E. Bond, et al., Nature 422: 405-408, March 27, 2003), the distance to V838 Mon is no less than 2000 pc (6520 light years), and they use a distance of 6000 pc (19,560 ly) to derive the brightness, giving an absolute visual magnitude Mv = -9.6. However, in the discovery paper (The mysterious eruption of V838 Mon, U. Munari et al., Astronomy and Astrophysics 389(2): L51-L56, July 2002) the distance is given as 790 +/- 30 pc (2575 +/- 98 ly), a lot closer than Bond et al. seem to think. We really need to know the true distance to get the true brightness, so I think this difference needs to be settled before we can be definitive about how bright it really was at peak.

However, even if it did peak at Mv = -9.6, it would not have been the brightest star in the Milky Way, as asserted by Bond, et al. Eta Carinae also sports Mv = -9.6, but Cyg OB2 #12 shines with Mv = -10.6, a full magnitude (about 2.5 times) brighter. The Pistol Star may well be brigher too, at Mv = -10.4, if the high temperature model is correct (but a wimpy Mv = -8.9 for the low temperature model). And there could easily be an even brighter star out there in the vast Milky Way somewhere; as individuals, we have seen hardly a miniscule fraction of the total number of roughly 10^11 or maybe 10^12 stars. So I'm not ready to buy the "brightest star" yet.

But whatever happened on V838 Mon, it appears to have been unique. According to Munari et al., it is a "new class of astronomical object", and I don't have any argument with that. The progenitor star was not that far along in its evolution (it was an F-class main sequence star), so it's not in the same class, for instance as Sakurai's Object. According to another paper that came out this year (Main-sequence stellar eruption model for V838 Monocerotis, N. Soker & R. Tylenda, Astrophysical Journal 582(2): L105-L108, Part 2, January 10, 2003), the merger of two main sequence stars, one about 1.5 solar masses, and the other 0.1 to 0.5 solar masses, is consistent with the observed phenomena. But Munari et al. indicate that the progenitor appeared to be an under-luminous F star, which seems unlikely if it was a binary, so maybe that argues against the merger hypothesis.

I guess the conclusion at this point is the one all scientists like to hear: More research is needed.

fando · 03-31-2003, 11:15 PM

Ah... I see. Time to start reading the actual papers again. Probably not the end life of a star as I guessed. It's easy to be mislead by watered down articles and hype.

ps418 · 04-01-2003, 07:05 AM

Interesting. So it is not clear whether the star is closer to 3 or to 20 thousand light-years from earth? You'd think that would have to be pinned down before Bond et al would make inferences about its absolute brightness! Friggin press releases!

Patrick

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03-31-2003, 09:45 AM	#2
ps418 Veteran Member Join Date: Mar 2001 Location: Louisville, KY, USA Posts: 1,840	Forgot to add: this star is on the cover of the latest issue of Nature, and the issue contains "letter to Nature" about the phenomena: Reflected glory: A stellar explosion like no other Patrick

03-31-2003, 10:07 AM	#3
Abacus Veteran Member Join Date: Mar 2002 Location: South Dakota Posts: 2,214	And it was the Astronomy Picture of the Day just a few days ago.

03-31-2003, 04:13 PM	#4
fando Veteran Member Join Date: Sep 2001 Location: Los Angeles Area Posts: 1,372	Nice! Let's see... this must be what happens to stars not massive enough to go nova but barely so. We're starting to fill in the grey areas of star evolution, or the missing links so to speak.

03-31-2003, 07:41 PM	#5
Tim Thompson Regular Member Join Date: Sep 2000 Location: Pasadena, CA, USA Posts: 455	According to the Nature paper (An energetic stellar outburst accompanied by circumstellar light echoes, Howard E. Bond, et al., Nature 422: 405-408, March 27, 2003), the distance to V838 Mon is no less than 2000 pc (6520 light years), and they use a distance of 6000 pc (19,560 ly) to derive the brightness, giving an absolute visual magnitude Mv = -9.6. However, in the discovery paper (The mysterious eruption of V838 Mon, U. Munari et al., Astronomy and Astrophysics 389(2): L51-L56, July 2002) the distance is given as 790 +/- 30 pc (2575 +/- 98 ly), a lot closer than Bond et al. seem to think. We really need to know the true distance to get the true brightness, so I think this difference needs to be settled before we can be definitive about how bright it really was at peak. However, even if it did peak at Mv = -9.6, it would not have been the brightest star in the Milky Way, as asserted by Bond, et al. Eta Carinae also sports Mv = -9.6, but Cyg OB2 #12 shines with Mv = -10.6, a full magnitude (about 2.5 times) brighter. The Pistol Star may well be brigher too, at Mv = -10.4, if the high temperature model is correct (but a wimpy Mv = -8.9 for the low temperature model). And there could easily be an even brighter star out there in the vast Milky Way somewhere; as individuals, we have seen hardly a miniscule fraction of the total number of roughly 10^11 or maybe 10^12 stars. So I'm not ready to buy the "brightest star" yet. But whatever happened on V838 Mon, it appears to have been unique. According to Munari et al., it is a "new class of astronomical object", and I don't have any argument with that. The progenitor star was not that far along in its evolution (it was an F-class main sequence star), so it's not in the same class, for instance as Sakurai's Object. According to another paper that came out this year (Main-sequence stellar eruption model for V838 Monocerotis, N. Soker & R. Tylenda, Astrophysical Journal 582(2): L105-L108, Part 2, January 10, 2003), the merger of two main sequence stars, one about 1.5 solar masses, and the other 0.1 to 0.5 solar masses, is consistent with the observed phenomena. But Munari et al. indicate that the progenitor appeared to be an under-luminous F star, which seems unlikely if it was a binary, so maybe that argues against the merger hypothesis. I guess the conclusion at this point is the one all scientists like to hear: More research is needed.

03-31-2003, 11:15 PM	#6
fando Veteran Member Join Date: Sep 2001 Location: Los Angeles Area Posts: 1,372	Ah... I see. Time to start reading the actual papers again. Probably not the end life of a star as I guessed. It's easy to be mislead by watered down articles and hype.

04-01-2003, 07:05 AM	#7
ps418 Veteran Member Join Date: Mar 2001 Location: Louisville, KY, USA Posts: 1,840	Interesting. So it is not clear whether the star is closer to 3 or to 20 thousand light-years from earth? You'd think that would have to be pinned down before Bond et al would make inferences about its absolute brightness! Friggin press releases! Patrick

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