Starship Asterisk*

Reflection Nebula vdB1

Explanation: Every book has a first page and every catalog a first entry. And so this lovely blue cosmic cloud begins the van den Bergh Catalog (vdB) of stars surrounded by reflection nebulae. Interstellar dust clouds reflecting the light of the nearby stars, the nebulae usually appear blue because scattering by the dust grains is more effective at shorter (bluer) wavelengths. The same type of scattering gives planet Earth its blue daytime skies. Van den Bergh's 1966 list contains a total of 158 entries more easily visible from the northern hemisphere, including bright Pleiades cluster stars and other popular targets for astroimagers. Less than 5 light-years across, VdB1 lies about 1,600 light-years distant in the constellation Cassiopeia. Also on this scene, two intriguing nebulae at the right show loops and outflow features associated with the energetic process of star formation. Within are extremely young variable stars V633 Cas (top) and V376 Cas.

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Ahh, Earth's blue daytime skies. That's something even i have a picture of. Although i could never seem to get an even shade of blue through the whole picture.

It's always great to see one of Adam Block's pictures here!

It's a lovely picture that shows several different kinds of nebulosity. The delightful blue reflection nebulosity is a galactic staple, fortunately! Also note the purplish streak to the right of the "blue fuzz", suggestive of emission nebulosity. The fascinating whitish shapes at right suggest low-mass star formation to me. They look "wild and imaginative", as if a cosmic Picasso had painted them on the easel of the night!

(And thank you, Beyond, for that picture of the beautiful blue sky of the Earth!)

Ann

shouldn't dust make light more red than blue?

Tsap wrote:shouldn't dust make light more red than blue?

Dust would, but not gas. Think of how a moonrise after a windy, dusty day (at least here in dry Texas) makes the moon look red, but with a clean sky it looks normal.

Ann wrote: "The fascinating whitish shapes at right suggest low-mass star formation to me." I don't know about the mass of the stars involved but there is star formation going on there, as discussed in the last link in the Expanation. What draws my eye about this apod is the extremely dark dust cloud that looks like a hole in space just to the left of the star forming region Ann mentioned. Must be a cold dense dust cloud, the remnant of the molecular cloud that the young stars in this whole area formed from.

Bruce

BDanielMayfield wrote:

Tsap wrote:shouldn't dust make light more red than blue?

Dust would, but not gas. Think of how a moonrise after a windy, dusty day (at least here in dry Texas) makes the moon look red, but with a clean sky it looks normal.

When a dust cloud is between us and a light source, the dust makes the light look redder. That is because red light penetrates the dust more easily than blue light, so more blue than red light is blocked by the dust, making the light look redder.

But if the light source is between us and the dust cloud, then some of the blue light that hits the dust cloud is reflected back at us, making the total illumination look bluer.

Ann

Thanks for the more complete explanation.

Bruce

Beautiful picture that makes me wonder. I'm trying to get my head around the nebula(s) on the right and what they may look like from another POV. Is there a tool (software, etc.) that might help us visualize the super structure of a formation as complicated as this?

Thank you for another fabulous image

.. with ghosts and ectoplasma as befits the season

sage wrote:

Yes this little nebula to the side steals the show... the turrets in the background scatter light like clouds on Earth with the sun behind them, glowing extra bright at the edges. The way the star (or stars) hidden behind the dark cloud seem just about to peek out is so remeniscient of a beautiful sunrise on Earth.

Perhaps there are two Earth-atmosphere like phenomena seen in this picture: the blue scattering of the main vdB1 and a crepuscular ray through the cloud on the right?

BDanielMayfield wrote:Ann wrote: "The fascinating whitish shapes at right suggest low-mass star formation to me." I don't know about the mass of the stars involved but there is star formation going on there, as discussed in the last link in the Expanation. What draws my eye about this apod is the extremely dark dust cloud that looks like a hole in space just to the left of the star forming region Ann mentioned. Must be a cold dense dust cloud, the remnant of the molecular cloud that the young stars in this whole area formed from.

Bruce

Good catch, Bruce. Yes, that extremely dark almost crescent-shaped cloud is undoubtedly the remnant of the dark molecular cloud that V633 Cas and V376 Cas were born from.

I want to add something about how dust affects the color of the light that reaches us from stars. Note the three bright stars within the blue reflection nebula in today's APOD. The three bright stars are all of spectral class B, and therefore they are all intrinsically blue. More specifically, they have B-V indexes that are negative, which means they emit more blue than yellow-green light.

I said that these stars have negative B-V indexes. In a way that isn't exactly true. Their intrinsic, unreddened B-V indexes are negative. But when we measure the B-V indexes of the light that reaches us directly from these stars, we find that these indexes are positive. This can mean only one thing: there was dust between the stars and the Earth, and this dust scattered some of the blue light away. Therefore the stars look redder, or at least yellower, than they really are.

What happened to the blue light that was scattered away by the dust between us and the stars? It kept being scattered by dust grains in the area. Some of that blue light was scattered our way. That's why we see an extended blue nebula, which surrounds a few intrinsically blue stars which look slightly yellowish.

What we see here actually explains why the sky here on the Earth is blue and the Sun looks yellow. In reality, the Sun is white. But much of the blue light from the Sun that enters the Earth's atmosphere is scattered in the atmosphere, making the sky look blue. For the same reason, because the light that reaches us directly from the Sun has had much of its blue component scattered away in the atmosphere, the concentrated, blindingly bright light from the Sun is yellower than the intrinsic color of the Sun. (In the picture you can see if you follow the link, the Sun is overexposed to a white color.)

Ann

Ann wrote: In reality, the Sun is white.
Ann

I thought the Sun is actually green.

Golganfinchian wrote:

Ann wrote: In reality, the Sun is white.
Ann

I thought the Sun is actually green.

It is! But it is white, too!

Here is how it works. We humans see the total light from the Sun (direct sunlight plus sky) as white. Our eyes are adapted to seeing sunlight as "neutral". It would probably be slightly stupid to see sunlight as colored, because then the background light (the sunlight) would always be colored, and then the colors of objects around us would have to compete with the background color of the Sun. It wouldn't be so smart.

However, the Sun is green. It is green because its maximum emission is in the green part of the spectrum. Interestingly, that means that other stars in the sky whose spectra resemble that of the Sun, such as Alpha Centauri, should in fact look green to us. But they don't, because our eyes are adapted to seeing light similar to sunlight as white.

That's why we see no green stars in the night sky!

Ann

Can someone explain
"Interstellar dust clouds reflecting the light of the nearby stars, the nebulae usually appear blue because scattering by the dust grains is more effective at shorter (bluer) wavelengths."
I must be misreading this. It sounds like the bluer wavelengths are causing the dust grains to scatter in a more effective pattern. What is this sentence trying to say?
Thanks
(PS: No blue skies here in Vancouver. )

BLazarus wrote:Can someone explain
"Interstellar dust clouds reflecting the light of the nearby stars, the nebulae usually appear blue because scattering by the dust grains is more effective at shorter (bluer) wavelengths."
I must be misreading this. It sounds like the bluer wavelengths are causing the dust grains to scatter in a more effective pattern. What is this sentence trying to say?
Thanks
(PS: No blue skies here in Vancouver. )

The dust grains are about the same size as the wavelength of the blue light. That means that when "a blue photon" intercepts a dust grain, it is very likely to "have its path blocked by the dust grain" and be scattered away by it. Imagine running full force into an obstacle. You will probably just fall down, but what if you bounced off of it? That is what happens to many of the blue photons.

The red photons represent longer wavelengths, and they are bigger than the dust grains. They can "force their way through" the dust almost like a snowplow.

Ann

sage wrote:Thank you for another fabulous image

.. with ghosts and ectoplasma as befits the season

Those caught my eye also. The cosmic Sheep Dog


with his friendly kitty under him

Ann wrote:
The dust grains are about the same size as the wavelength of the blue light. That means that when "a blue photon" intercepts a dust grain, it is very likely to "have its path blocked by the dust grain" and be scattered away by it. Imagine running full force into an obstacle. You will probably just fall down, but what if you bounced off of it? That is what happens to many of the blue photons.

The red photons represent longer wavelengths, and they are bigger than the dust grains. They can "force their way through" the dust almost like a snowplow.

Ann

I get it now. Thanks

Golganfinchian wrote:

Ann wrote: In reality, the Sun is white.
Ann

I thought the Sun is actually green.

So, what does this make the sun really, a white dwarf??? or a green dwarf??? Really now, why should a middle of the main sequence middle aged star be called a dwarf simply because it's smaller than over the hill bloated behemoths? Therefore, I say, down with dwarfism in astronomy! Replanitize Pluto! Dedwarf the Sun!

Bruce

Ann,

Not all dust scatters light the same way. On Mars (and sometimes on Earth), the sun looks blue through dust.

Golganfinchian wrote:I thought the Sun is actually green.

The Sun is not green. It is white. It's quite impossible for any star to be green, since no blackbody can produce green light.

You are perhaps confusing the idea that the Sun is green with the fact that its peak energy output is in the green part of the spectrum. But those are completely different things.

BDanielMayfield wrote:

Golganfinchian wrote:

Ann wrote: In reality, the Sun is white.
Ann

I thought the Sun is actually green.

So, what does this make the sun really, a white dwarf??? or a green dwarf??? Really now, why should a middle of the main sequence middle aged star be called a dwarf simply because it's smaller than over the hill bloated behemoths? Therefore, I say, down with dwarfism in astronomy! Replanitize Pluto! Dedwarf the Sun!

Bruce

The terminology could actually kill you!!!

Stars are called "dwarfs" when they are on the so called main sequence. When stars are on the main sequence, they fuse hydrogen to helium in their cores. There are "yellow dwarfs" like G-type Alpha Centauri A and the Sun (even though both Alpha Centauri A and the Sun are white), "orange dwarfs" like Alpha Centauri B (the K-type yellow star which was recently found to have a planet) and "red dwarfs" like tiny Proxima Centauri, the tiny M-type yellow-orange star which is at present the star other than the Sun which is closest to the Earth. You can see the relative sizes and colors of the Sun, Alpha Centauri A, Alpha Centauri B and Proxima Centauri here.

We may even talk about "blue dwarfs". Regulus, the B-type blue-white alpha star of constellation Leo can be described as a blue dwarf, because it is still on the main sequence and fuses hydrogen to helium in its core. Regulus is of spectral class B7V (the "V" means "main sequence"). A bigger, hotter star which is also on the main sequence is Achernar, alpha star of the constellation Eridanus. Achernar is of spectral class B3V. Interestingly, both Regulus and Achernar are highly oblate due to very fast rotation. You can see a comparison between the Sun, A-type dwarf Altair and B-type dwarfs Regulus and Achernar here.

All stars as massive as the Sun, and probably all stars as massive as Alpha Centauri B, are destined to eventually use up the hydrogen in their cores. Then they can't fuse hydrogen to helium in their cores any more, and then they start fusing hydrogen to helium in a shell around their cores instead. Then they are no longer dwarfs. Instead, they have become become giants.

Okay. You can have red, orange, yellow and blue dwarfs. You can also have red, orange, yellow, white or blue giants. But what about the main sequence stars which are technically described as white, even though they are blue-white? What about Vega? It is a main sequence star of spectral class A0. Since it belongs to spectral class A, it may technically be described as "white", in the same way that the Sun is described as "yellow". And since it fuses hydrogen to helium in its core, it is a dwarf. So Vega is a "white dwarf", then?

Sadly, no! Because "white dwarf" is a term that refers to the tiny exposed cores of dead stars. White dwarfs start out as central stars of planetary nebulae. Check out this picture of a planetary nebula. Note the very blue color of the central star. This very blue star is a white dwarf!!!

Ann

Ah, a little nuclear astrophysics for breakfast! Yum. Thanks Ann. The terminology won’t kill me but it does tick me off, and as I read through your delightful post I got even more steamed. So even stars like Achernar are dwarves!? (dwarfs? Which is correct? I’ve read scientific papers where the later is used, but it doesn’t sound right.) Achernar as a dwarf is totally absurd. So, space is a realm where there are no average stars, but everyone is either a dwarf, a giant, or a super-giant? How ridiculous!

(For any wonderful little people who may read this rant, I intend no offence. Being somewhat of a giant myself [I’m 6’5”] I envy the fact that ya’ll never have to experience the pain of bashing your head against door frames, light fixtures, ceiling beams, tree limbs, etc., etc., etc.)

More seriously now, I think that in view of the confusion that overuse of the modifier “dwarf” in stellar terminology has led to, it would be better to reserve the term for stars that are actually small, such as white dwarfs and red dwarfs below the bend in the main sequence on H-R diagrams. Except for red dwarfs, hydrogen burning typical stars should be called what they actually are; i.e. our sun is a yellow main sequence star.

However, since my wishes here mean absolutely nothing, In way of protest I will as an amateur astronomer henceforth take this dwarf terminology convention as license to call whomever I wish, no matter how tall they may actually be, a dwarf. (Unless they actually are diminutive, whereupon the p.c. “little person” is preferred.) This will be most vigorously utilized whenever someone asks me “How’s the weather up there?” to which I’ll respond, “Fine. What’s it like being a dwarf?” or the equivalent. You’ve been warned world.

Bruce, a GIANT among the dwarves.

Ann wrote:The terminology could actually kill you!!!

Stars are called "dwarfs" when they are on the so called main sequence. When stars are on the main sequence, they fuse hydrogen to helium in their cores. There are "yellow dwarfs" like G-type Alpha Centauri A and the Sun (even though both Alpha Centauri A and the Sun are white), "orange dwarfs" like Alpha Centauri B (the K-type yellow star which was recently found to have a planet) and "red dwarfs" like tiny Proxima Centauri, the tiny M-type yellow-orange star which is at present the star other than the Sun which is closest to the Earth. You can see the relative sizes and colors of the Sun, Alpha Centauri A, Alpha Centauri B and Proxima Centauri here.

We may even talk about "blue dwarfs". Regulus, the B-type blue-white alpha star of constellation Leo can be described as a blue dwarf, because it is still on the main sequence and fuses hydrogen to helium in its core. Regulus is of spectral class B7V (the "V" means "main sequence"). A bigger, hotter star which is also on the main sequence is Achernar, alpha star of the constellation Eridanus. Achernar is of spectral class B3V. Interestingly, both Regulus and Achernar are highly oblate due to very fast rotation. You can see a comparison between the Sun, A-type dwarf Altair and B-type dwarfs Regulus and Achernar here.

All stars as massive as the Sun, and probably all stars as massive as Alpha Centauri B, are destined to eventually use up the hydrogen in their cores. Then they can't fuse hydrogen to helium in their cores any more, and then they start fusing hydrogen to helium in a shell around their cores instead. Then they are no longer dwarfs. Instead, they have become become giants.

Okay. You can have red, orange, yellow and blue dwarfs. You can also have red, orange, yellow, white or blue giants. But what about the main sequence stars which are technically described as white, even though they are blue-white? What about Vega? It is a main sequence star of spectral class A0. Since it belongs to spectral class A, it may technically be described as "white", in the same way that the Sun is described as "yellow". And since it fuses hydrogen to helium in its core, it is a dwarf. So Vega is a "white dwarf", then?

Sadly, no! Because "white dwarf" is a term that refers to the tiny exposed cores of dead stars. White dwarfs start out as central stars of planetary nebulae. Check out this picture of a planetary nebula. Note the very blue color of the central star. This very blue star is a white dwarf!!!

Ann

While dwarf may traditionally apply to main sequence stars, you won't hear of stars above class G referred to as [color] dwarf. As Ann has noted, white dwarf has a special meaning, so you'll never hear of Polaris (class F) being referred to as a white dwarf. Nor will you hear of OB stars, such as Rigel, being referred to as blue dwarfs. In fact, as with white dwarf, blue dwarf has a special meaning. A blue dwarf is a hypothetical late stage in the life of a red dwarf.

bystander wrote:
While dwarf may traditionally apply to main sequence stars, you won't hear of stars above class G referred to as [color] dwarf. As Ann has noted, white dwarf has a special meaning, so you'll never hear of Polaris (class F) being referred to as a white dwarf. Nor will you hear of OB stars, such as Rigel, being referred to as blue dwarfs. In fact, as with white dwarf, blue dwarf has a special meaning. A blue dwarf is a hypothetical late stage in the life of a red dwarf.

According to http://en.wikipedia.org/wiki/Blue_dwarf, the expression "blue dwarf" may refer to "An early-type main sequence star". "An early-type star" usually refers to a star of spectral class O or B, but sometimes refers to stars as cool as spectral class F5.

So not only could B7V star Regulus of "blue" spectral class B be described as a blue dwarf, but A0V star Vega of "white" spectral class A might actually be called a blue dwarf, too.

Obviously Rigel is not a dwarf, blue or otherwise. It is classified as a (blue-white) supergiant of spectral type B8Ia.

Ann