APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

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Expand view Topic review: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Astronomy Wannabe » Thu Jun 18, 2015 4:24 pm

Nitpicker wrote:... heliacal rising ...
Thanks, Nitpicker for introducing me to a new term! This is one I had not heard before, I found the Wikipedia explanation fascinating. https://en.wikipedia.org/wiki/Heliacal_rising

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Nitpicker » Thu Jun 18, 2015 12:51 am

Gosh, I had no idea of the ramifications of my calamari habit. (Thanks Chris.)

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Chris Peterson » Thu Jun 18, 2015 12:41 am

Nitpicker wrote:The "darkness" link leads to this APOD from 2001:
http://apod.nasa.gov/apod/ap010827.html

What (on Earth) is/was causing so much light pollution north of the Falkland Islands?
A huge Japanese fishing fleet that uses bright lights to attract squid.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Nitpicker » Thu Jun 18, 2015 12:27 am

The "darkness" link leads to this APOD from 2001:
http://apod.nasa.gov/apod/ap010827.html

What (on Earth) is/was causing so much light pollution north of the Falkland Islands?

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by geckzilla » Wed Jun 17, 2015 10:33 pm

Chris Peterson wrote:I see eight stars under good conditions, six under slightly poorer conditions. This is pretty typical. Nobody sees seven. The story talks about "Indians", but there are many different American Indian cultures, with many different stories about the stars, and a number of those stories feature six stars in the Pleiades.

It's unlikely that one star has dimmed in recent times. More likely is the fact that the number seven has numerological significance in many cultures, so the Pleiades got tweaked by culture, not nature.
I was reading about the star lore of various cultures the other night. A lot of myths and folklore have been lost as cultures have died off but it seems like the story of the Pleiades is one of the most commonly surviving legends from each culture. They've certainly made an impact upon a lot of humans throughout history.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Nitpicker » Wed Jun 17, 2015 10:18 pm

Also known as Matariki in New Zealand, where its heliacal rising (a few weeks ago) is considered to mark the Maori New Year:

https://en.wikipedia.org/?title=Matariki

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by hwelborn » Wed Jun 17, 2015 7:56 pm

I understand in Japan it is known as Subaru. How lovely!

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Chris Peterson » Wed Jun 17, 2015 7:37 pm

sambrewster wrote:Some of these remarkable pictures leave neophytes like myself thirsting for just a little more immediate information. In the instance of this APOD, the pronunciation of "Pleiades" plus a line about what makes up the dust would have hit the mark without, I believe, overly dumbing down the content. SB
There's no strictly correct pronunciation for most astronomical objects. The Pleiades is usually pronounced PLEE-uh-deez or PLAY-uh-deez.

Cosmic dust is made up of mainly of silicates and carbonates, ranging in size from single molecules to sub-micron conglomerations. The Wikipedia entry on the subject is quite good.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by sambrewster » Wed Jun 17, 2015 7:11 pm

Some of these remarkable pictures leave neophytes like myself thirsting for just a little more immediate information. In the instance of this APOD, the pronunciation of "Pleiades" plus a line about what makes up the dust would have hit the mark without, I believe, overly dumbing down the content. SB

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Beyond » Wed Jun 17, 2015 5:35 pm

Well, how about that. I came, i saw, i didn't recognize what i saw, the way i saw it. Thanks for pointing it out more distinctly, Chris.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Chris Peterson » Wed Jun 17, 2015 4:40 pm

Beyond wrote:Yeah, in the picture you posted, a blind man could see it, but i don't see it in the APOD picture. Maybe you didn't mean that one could see it in today's APOD, even though it's there??
knot.jpg

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Ann » Wed Jun 17, 2015 4:40 pm

Beyond wrote:
Ann wrote:The knot is at 7 o'clock, right below the brilliant star between two diffraction spikes.

Ann
Yeah, in the picture you posted, a blind man could see it, but i don't see it in the APOD picture. Maybe you didn't mean that one could see it in today's APOD, even though it's there??
You actually can see it in today's APOD, but it looks just like a little appendix hanging down from the bright star.

Ann

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Beyond » Wed Jun 17, 2015 4:18 pm

Ann wrote:The knot is at 7 o'clock, right below the brilliant star between two diffraction spikes.

Ann
Yeah, in the picture you posted, a blind man could see it, but i don't see it in the APOD picture. Maybe you didn't mean that one could see it in today's APOD, even though it's there??

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Chris Peterson » Wed Jun 17, 2015 2:39 pm

henrystar wrote:Epsilon Lyrae is a very nice test! When I was younger, I could certainly resolve it.
Do you wear glasses? Try tipping them up just a bit (raise the temples), which will increase the power slightly. You might find you can still resolve epsilon Lyra (into two, not four!)

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Chris Peterson » Wed Jun 17, 2015 2:35 pm

AndrewMGarland wrote:http://hypertextbook.com/facts/1999/MesahHarwood.shtml
Best laboratory vacuum 10^-16 g/cm3 (10^-10 g/m3)
Density of air at STP 1.29 × 10-3 g/cm3 (1.3 x 10^3 g/m3)

https://en.wikipedia.org/wiki/Cosmic_dust
Most cosmic dust varies between a few molecules up to 0.1 µm in size.

The dust density in the local interstellar medium is approximately
10^-6 × dust grain/m3. Each grain is approx 10^-14 g.
(10^-20 g/m3)

Summary:
Best lab vacuum: 10^-10 g/m3. Interstellar dust: 10^-20 g/m3
So, the lab vacuum is 10 billion times more dense than an interstellar dust cloud!

Are these estimates correct? What is the mass density outside of dust clouds, in the clear parts of interstellar space?
The local interstellar medium is currently somewhat denser than other areas of interstellar space, but nowhere near as dense as interstellar dust clouds. Thin clouds like that around the Pleiades are probably denser by several orders of magnitude, while dense clouds in star forming regions are many orders of magnitude denser.

Also, we have to be careful with the idea of "density" here. Your calculations rely on mass density, but particle density is also an important concept. The mass in hard laboratory vacuums comes from gas molecules, not dust particles. So they can have a much higher particle count for a given mass density. The optical effects we observe in astronomical nebulas are associated with the particle density, not the mass (but the mass density is important for understanding star formation).

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by henrystar » Wed Jun 17, 2015 2:31 pm

Epsilon Lyrae is a very nice test! When I was younger, I could certainly resolve it.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Chris Peterson » Wed Jun 17, 2015 2:27 pm

ceelias wrote:This may be of interest to some. Note that this is qouted from a different era.

From "The Book of Woodcraft and Indian Lore" published in `1929 by Ernest Thompson Seton:

"PLEIADES AS A TEST OF EYESIGHT

This star group has always been considered a good test of eyesight.
I once asked a group of boys in camp how many of the Pleiades they could count with the naked eye. A noisy, forward boy, who was nicknamed "Bluejay", because he was so fond of chattering and showing off, said "Oh, I see hundreds."
"Well, you can sit down,: I said, "for you can do nothing of the kind."
Another steadier boy said" I believe I see six," and he proved that he did see tem, for he mapped them out properly on a board with six pebbles.
That boy had good eyes, because poor eyes see merely a haze, but another boy present had better eyes, for he saw, and proved that he saw, seven. This is considered first class. The Indians as a rule see seven, because they call them the Seven Stars. But according to Flammarion, it is possible to exceed this, for several persons have given proof that they distinguished ten Pleiades. This is almost the extreme of human eyesight. There is however, according to the same authority, a record of thirteen Pleiades having been actually seen by the unaided human eye."
I see eight stars under good conditions, six under slightly poorer conditions. This is pretty typical. Nobody sees seven. The story talks about "Indians", but there are many different American Indian cultures, with many different stories about the stars, and a number of those stories feature six stars in the Pleiades.

It's unlikely that one star has dimmed in recent times. More likely is the fact that the number seven has numerological significance in many cultures, so the Pleiades got tweaked by culture, not nature.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by henrystar » Wed Jun 17, 2015 1:57 pm

The nebulosity might make you think that it is material left over from the formation of the star cluster, but that is not so. The cluster is just moving through the interstellar medium, and it naturally illuminates what it is passing through.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Ann » Wed Jun 17, 2015 12:55 pm

The knot is at 7 o'clock, right below the brilliant star between two diffraction spikes.

Ann

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Beyond » Wed Jun 17, 2015 10:55 am

Ann wrote: You can even make out the small knot in the Merope Nebula in today's APOD. This small knot is called IC 349, and it shows up well in this picture by Ivan Eder.

Ann
I'm not seeing the knot. Even up to 300% screen magnification, looking all around merope. Either I'm almost blind-as-a-bat, or I'm just not noticing the knot.

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by starsurfer » Wed Jun 17, 2015 10:54 am

This is a magnificent image of a superb cluster, you wouldn't think it was taken from the southern hemisphere!

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Guest » Wed Jun 17, 2015 10:39 am

AndrewMGarland wrote: The dust density in the local interstellar medium is approximately
10^-6 × dust grain/m3. Each grain is approx 10^-14 g.
(10^-20 g/m3)
So we did some math this morning and came up with a density of 8.85 x 10^24 KG/cubic-lightyear in interstellar space. (Round-off error in the units is assumed) Unless we made a mistake somewhere. It made me wonder just how dense various star forming regions have to be in order to collapse and create something like our sun at about 2 x 10^30 KG. Clearly cubic light years of material must collapse on itself, and we also wondered about the time needed for that process too...

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by ceelias » Wed Jun 17, 2015 10:33 am

This may be of interest to some. Note that this is qouted from a different era.

From "The Book of Woodcraft and Indian Lore" published in `1929 by Ernest Thompson Seton:

"PLEIADES AS A TEST OF EYESIGHT

This star group has always been considered a good test of eyesight.
I once asked a group of boys in camp how many of the Pleiades they could count with the naked eye. A noisy, forward boy, who was nicknamed "Bluejay", because he was so fond of chattering and showing off, said "Oh, I see hundreds."
"Well, you can sit down,: I said, "for you can do nothing of the kind."
Another steadier boy said" I believe I see six," and he proved that he did see tem, for he mapped them out properly on a board with six pebbles.
That boy had good eyes, because poor eyes see merely a haze, but another boy present had better eyes, for he saw, and proved that he saw, seven. This is considered first class. The Indians as a rule see seven, because they call them the Seven Stars. But according to Flammarion, it is possible to exceed this, for several persons have given proof that they distinguished ten Pleiades. This is almost the extreme of human eyesight. There is however, according to the same authority, a record of thirteen Pleiades having been actually seen by the unaided human eye."

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by AndrewMGarland » Wed Jun 17, 2015 7:52 am

http://hypertextbook.com/facts/1999/MesahHarwood.shtml
Best laboratory vacuum 10^-16 g/cm3 (10^-10 g/m3)
Density of air at STP 1.29 × 10-3 g/cm3 (1.3 x 10^3 g/m3)

https://en.wikipedia.org/wiki/Cosmic_dust
Most cosmic dust varies between a few molecules up to 0.1 µm in size.

The dust density in the local interstellar medium is approximately
10^-6 × dust grain/m3. Each grain is approx 10^-14 g.
(10^-20 g/m3)

Summary:
Best lab vacuum: 10^-10 g/m3. Interstellar dust: 10^-20 g/m3
So, the lab vacuum is 10 billion times more dense than an interstellar dust cloud!

Are these estimates correct? What is the mass density outside of dust clouds, in the clear parts of interstellar space?

Re: APOD: M45: The Pleiades Star Cluster (2015 Jun 17)

by Ann » Wed Jun 17, 2015 6:33 am

You can even make out the small knot in the Merope Nebula in today's APOD. This small knot is called IC 349, and it shows up well in this picture by Ivan Eder.

Ann

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