[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: [APML] OT: A Deep M13
Hi Robert,
>Hi all,
>Here's an M13 collaboration, data by Jim Misti, processed by me.
>
>http://www.robgendlerastropics.com/M13JM.html
>
>Just a little clarification about the presence of blue stars in many
>globulars. Many of us have erroneously attributed the blue population
>stars to "Blue Stragglers" which have received lots of publicity recently.
>Probably the vast majority of blue stars in globulars are blue horizontal
>branch stars which having exhausted their hydrogen fuel are now
>fusing helium to carbon. The other bland colored or white stars are mostly
>RR Lyrae stars which surprisingly make up a substantial population of many
>globulars. The blue stragglers which are likely the result of either
>collisions
>in high density central regions or mass transfer in binary systems are
>considerably
>dimmer than the helium burning horizontal branch stars and are mostly
>located
>in the central dense core of globulars.
>
>In summary many of the blue stars we see in CCD images of
>globulars are blue horizontal branch helium burning stars and not
>blue stragglers which are mostly central and less luminoius than the
>other populations mentioned.
>
>I hope that professional astronomers out there will let me know if I got
>some of this wrong.
>
>
>
As it so happens, I am a professional astronomer and I specialise in
observing globular clusters! But don't worry, nearly everything you said
above was kosher. I'd only make a few modifications for accuracy; and
while I'm at it, I'll add some additional information for those who may
be curious about globular cluster stars.
- The horizontal branch (HB) stars which you said have "exhausted their
hydrogen fuel" have only exhausted it in their _cores_. They still have
plenty of it in their outer parts (envelope), although they have also
lost a considerable fraction of their envelope mass due to their strong
stellar winds in the red giant branch (RGB) phase, which just precedes
the HB phase. While these strong winds are carrying gas away from the
surface of the red giant, its core is contracting, getting denser and
hotter and producing far more energy; meanwhile its atmosphere is
swelling and cooling, so it moves up the RGB, getting both brighter and
cooler/redder. Once up at the tip of the RGB, there is a sudden "Helium
Flash" - the ignition of helium burning in the core - and the star
rapidly jumps to the HB. On the HB, it resumes a stable configuration
for a while - sort of like the Main Sequence, but with helium burning in
the core instead of hydrogen burning. But here's the interesting bit -
the extent of the mass loss as a red giant appears to be the biggest
factor in accounting for where exactly it jumps to on the HB.
- While it is true that "the other bland colored or white stars" are
often RR Lyrae stars, I wouldn't say that they are "mostly" RR Lyrae
stars. In most tri/quadcolour images such as yours, only the brighter
red giants are obviously red/orange/yellow, and only the blue horizontal
branch (BHB) stars are obviously blue. Everything else, that is the bulk
of the cluster's resolved stars, falls in the middle, and appears
"white". This includes the middle of the HB (where the RR Lyrae stars
reside), the red end of the HB (the RHB), the dimmer parts of the giant
branch (the subgiants branch or SGB), and the main sequence turn off
(MSTO) . These latter two populations are far more numerous than the
others I mentioned, since stars spend more of their lives in those phases.
- Although they encompass a wide range of temperatures and hence
colours, most blue straggler stars (BSS) are not particularly blue
(hot); they typically have the same colour as the middle of the HB & the
RR Lyrae stars. They earned the monicker "blue" from simply lying
bluewards of the MSTO, as though they were a weird extension of the MS
to brighter, hotter MS stars - which was considered impossible if they
were as old as all the other stars in the cluster. Such hotter/more
massive "MS" stars _should_ have evolved off the MS, and become red
giants and HB stars a long time ago - but they seem to have been left
behind by their peers, hence they are "stragglers". So the only way to
explain their presence is if they are indeed "younger" stars, formed by
some extreme mass transfer process like a collision or a gradual
coalsecence of two smaller stars.
- There are two reasons why BSS (or any other type of star) should
appear to be centrally concentrated in the core of a cluster. The first
is that if they are the products of a collision, the greater crowding of
stars makes a collision in the core more likely. The second reason is
that any heavier stars - more massive ones such as coalesced BSS, binary
stars (combined mass of two stars), or old heavy remnants like the
neutron stars left behind by the long dead massive stars that the
cluster had in its youth - tend to sink towards the centre of the
cluster due to a process called "mass segregation". In gravitational
encounters between a massive star and a less massive one, the massive
one will lose some kinetic energy and the smaller one will gain it, so
the orbit of the massive one within the cluster shrinks while the
smaller one is thrown out into a wider orbit. (This is the same effect
used to give gravitational boosts to planetary spacecraft like Voyager,
Galileo and Cassini).
Ray "who loves to teach this sort of thing, and could go on and on..."
Butler.
>Rob Gendler
>email: robgendler@att.net
>Web Site: http://www.robgendlerastropics.com
>
>_______________________________________________
>Astro-Photo mailing list
>Astro-Photo@seds.org
>http://seds.org/mailman/listinfo/astro-photo
>
>
--
Dr. Ray Butler
Lecturer, Physics & Astronomy | Physics Department | Computational Astrophysics Laboratory
National University of Ireland - Galway, University Road, Galway, Ireland | www.nuigalway.ie
ray.butler-AT-nuigalway.ie | Tel: +353-91-524411 ext 3788 | FAX: +353-91-750584
_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo