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

[APOD Robot]

M45: The Pleiades Star Cluster

Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as dusty as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The featured exposure took over 12 hours and covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

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[Beyond]

Ann's favorite place.

[Nitpicker]

That's a great image ... and very high resolution, too. Though I might have said the lower resolution APOD from 2014-02-25 -- http://apod.nasa.gov/apod/ap140225.html -- was dustier.

If you prefer annotations of star names, over dust, then this noisy one of mine shows the names of the seven sisters, along with mum and dad on the left. It was taken lazily, without a telescope, just a DSLR, 300mm FL, f/5.6, very late in the season with M45 about to set in evening twilight:

[Ann]

Ann's favorite place.

You got it, Beyond!!!

Ann

[Boomer12k]

I like the clarity of the different direction of striation...

:---[===] *

[Ann]

Click to view full size image

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

[AndrewMGarland]

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?

[ceelias]

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."

[Guest]

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...

[starsurfer]

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

[Beyond]

Click to view full size image

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.

[Ann]

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

Ann

[henrystar]

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.

[Chris Peterson]

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.

[henrystar]

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

[Chris Peterson]

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).

[Chris Peterson]

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!)

[Beyond]

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??

[Ann]

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

[Chris Peterson]

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??

[Beyond]

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.

[sambrewster]

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

[Chris Peterson]

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.

[hwelborn]

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

[Nitpicker]

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