APOD: Stars of a Summer's Triangle (2015 Jun 27)

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APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by APOD Robot » Sat Jun 27, 2015 4:08 am

Image Stars of a Summer's Triangle

Explanation: Rising at the start of a northern summer's night, these three bright stars form the familiar asterism known as the Summer Triangle. Altair, Deneb, and Vega are the alpha stars of their respective constellations, Aquila, Cygnus, and Lyra, nestled near the Milky Way. Close in apparent brightness the three do look similar in these telescopic portraits, but all have their own stellar stories. Their similar appearance hides the fact that the Summer Triangle stars actually span a large range in intrinsic luminosity and distance. A main sequence dwarf star, Altair is some 10 times brighter than the Sun and 17 light-years away, while Vega, also a hydrogen-fusing dwarf, is around 30 times brighter than the Sun and lies 25 light-years away. Supergiant Deneb, at about 54,000 times the solar luminosity, lies some 1,400 light-years distant. Of course, with a whitish blue hue, the stars of the Summer Triangle are all hotter than the Sun.

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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by Guest » Sat Jun 27, 2015 9:31 am

I know these estimates constantly change; but the last I heard, Deneb was estimated to be about 2,600 light-years way and about 125,000 solar luminosties. Are the estimates you have on this page new ones or old ones?

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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by Nuther Guest » Sat Jun 27, 2015 11:47 am

Being past 70, maybe it's time I learnt to recognize
that bl**dy summer triangle at last !

Please kind Apod regulars, help me find them stars,
what is the approx. angular size of the asterism ?

If I'm not mistaken, it would culminate around
the local midnight at this time of the year, right ?

Congrats to all of you who make those wondeful photos
available for our eyes' pleasure !

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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by bystander » Sat Jun 27, 2015 2:16 pm

Nuther Guest wrote:Being past 70, maybe it's time I learnt to recognize
that bl**dy summer triangle at last ! ...

Try the annotated image at APOD: Summer Triangles over Japan (2015 May 06). It may help.
There is also a link in its explanation that might help: Summer Triangle: Vega, Deneb, Altair.
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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by starsurfer » Sat Jun 27, 2015 3:15 pm

This is a great and interesting way to present what is arguably one of the most recognisable asterisms in the sky. I would love to see a mosaic of the whole constellation of Aquila done by Rogelio Bernal Andreo in the same way he did Orion.

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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by ta152h0 » Sat Jun 27, 2015 10:59 pm

I have seen images of the sun where astronomers have superimposed disks representing the actual size on the sun. Are these stars too far away to do the same ?
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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by ta152h0 » Sat Jun 27, 2015 11:03 pm

Ever seen the Southern Cross ? that is a' in-your-face " asterism, along with Orion.
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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by Chris Peterson » Sat Jun 27, 2015 11:24 pm

ta152h0 wrote:I have seen images of the sun where astronomers have superimposed disks representing the actual size on the sun. Are these stars too far away to do the same ?
These stars are essentially point sources. Their actual sizes at these image scales are much less than a single pixel. They only appear to have a finite diameter because of diffraction and optical aberrations.
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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by Kaius » Mon Jun 29, 2015 7:29 am

Chris Peterson wrote: These stars are essentially point sources. Their actual sizes at these image scales are much less than a single pixel. They only appear to have a finite diameter because of diffraction and optical aberrations.
Hi Chris,
As you said, stars are point sources and are blurred by diffraction effects. But I have been wondering why it is possible to see dust disks and exoplanets by blocking out the star's light using a coronagraph like what we typically do with the sun, as the picture below shows. I tend to think that the diffracted light of the star will be mixed up with that from the disk, and there is no way to separate them. Is this kind of blocking done before diffraction takes place?

Image

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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by Chris Peterson » Mon Jun 29, 2015 1:34 pm

Kaius wrote:
Chris Peterson wrote: These stars are essentially point sources. Their actual sizes at these image scales are much less than a single pixel. They only appear to have a finite diameter because of diffraction and optical aberrations.
Hi Chris,
As you said, stars are point sources and are blurred by diffraction effects. But I have been wondering why it is possible to see dust disks and exoplanets by blocking out the star's light using a coronagraph like what we typically do with the sun, as the picture below shows. I tend to think that the diffracted light of the star will be mixed up with that from the disk, and there is no way to separate them. Is this kind of blocking done before diffraction takes place?
Well, a simple occulting disk creates its own diffraction. You can't fool Mother Nature.

In practice, a good deal of the bloating we see in overexposed stars is caused not by diffraction, but by internal scatter in the optics. Coronagraphs are designed to minimize such scatter. And the coronagraphs used for imaging exoplanets aren't simple shadow masks, but complex optical elements that alter the polarization and phase of incoming light to block the undesired elements.
Chris

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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by Kaius » Thu Jul 02, 2015 3:32 pm

Hi, Chris,

Thanks for your timely reply. To be honest I have never heard of scattering as a factor of bloating :? . Gonna do more research on this :D .

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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by ta152h0 » Thu Jul 02, 2015 8:07 pm

how big is a pixel ?
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Re: APOD: Stars of a Summer's Triangle (2015 Jun 27)

Post by BMAONE23 » Thu Jul 02, 2015 9:13 pm

ta152h0 wrote:how big is a pixel ?
Image
About that big