APOD: Hydrogen in the LMC (2011 Apr 26)

[APOD Robot]

Hydrogen in the LMC

Explanation: A satellite galaxy of our own Milky Way, the Large Magellanic Cloud (LMC) is an alluring sight in dark southern skies and the constellation Dorado. A mere 180,000 light-years distant, the LMC is seen in amazing detail in this very deep 4 frame mosaic of telescopic images, a view that reveals the Milky Way's satellite to have the appearance of a fledgling barred spiral galaxy. The mosaic includes image data taken through a narrow filter that transmits only the red light of hydrogen atoms. Ionized by energetic starlight, a hydrogen atom emits the characteristic red H-alpha light as its single electron is recaptured and transitions to lower energy states. As a result, this mosaic seems spattered with pinkish clouds of hydrogen gas surrounding massive, young stars. Sculpted by the strong stellar winds and ultraviolet radiation, the glowing hydrogen clouds are known as H II (ionized hydrogen) regions. Composed of many overlapping clouds, the sprawling Tarantula Nebula left of center, is by far the LMC's largest star forming region. The Large Magellanic Cloud is about 15,000 light-years across.

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[orin stepanek]

A beautiful little barred spiral is the LMC! It's star forming region; the Tarantula is also neat.

[owlice]

I love the Tarantula Nebula (tarantulas, not so much!), and would like to have a mural made of this image of it to put in my house:

Click to view full size image

(Hmmmm... I'm dressed to match that image today. Maybe I should take this as a sign and order the mural! )

[Skooterp]

Are all the little spots stars in the foreground, or galaxies in the background?
I know the really bright ones are stars within our own galaxy, but I'm not so sure about the little spots that cover the rest of the image.

Thanks!

[CWDavis]

Very neat! But, is the LMC a "fledgling barred spiral galaxy", which implies to this novice that it may split off from the Milky Way, or is it a captured galaxy that may be in the process of 'dissipating'?

[NoelC]

The Large Magellanic Cloud is about 15,000 light-years across.

Otto (or others), do you know an estimate of the number of stars in there?

I'm thinking whatever civilizations lie in that mass of stars probably have a heckuva view of our grand spiral. What a great place from which to study galactic mechanics!

I'm imagining societies so advanced they have built observatories the size of solar systems with which to study the cosmos. They may well be able to see our nude sunbathers on building tops from there... Er, they will be in 160K years or so.

-Noel

[neufer]

Are all the little spots stars in the foreground, or galaxies in the background?
I know the really bright ones are stars within our own galaxy, but I'm not so sure about the little spots that cover the rest of the image.

The glowing hydrogen clouds are known as H II (ionized hydrogen) regions.

[garry]

What exactly is " ionized by energetic starlight"? I mean are the photons that strong? If energetic starlight is this high powered light? If ionized does this mean by radiation, plasma, gas, what? It is not a very god explanation from a scientific viewpoint!

[neufer]

What exactly is " ionized by energetic starlight"?

I mean are the photons that strong?

Yes.

Applying "Wien's Law":



gives a star temperature of ~ 31,000 K to produce its
peak black body radiation at the 91.2 nm required to ionize hydrogen.

Stars much cooler than this will also do a good job at ionization.

[Chris Peterson]

What exactly is " ionized by energetic starlight"? I mean are the photons that strong? If energetic starlight is this high powered light? If ionized does this mean by radiation, plasma, gas, what? It is not a very god explanation from a scientific viewpoint!

Photons are a fundamental carrier of energy. They are what cause ionization, by knocking electrons off of atoms. Photon radiation is the most common type of radiation- aka light. Plasma and gas can't ionize anything. A plasma is an ionized gas- usually ionized by photons.

[NoelC]

Handy ionized hydrogen gas identification guide.

-Noel

[Ann]

What exactly is " ionized by energetic starlight"?

I mean are the photons that strong?

Yes.

Applying "Wien's Law":



gives a star temperature of ~ 31,000 K to produce its
peak black body radiation at the 91.2 nm required to ionize hydrogen.

Stars much cooler than this will also do a good job at ionization.

True, but stars cooler than what is typical for, say, stars of spectral type B2, will not do a good job at ionization.

Take a look at this image. It shows the Antares and Rho Ophiuchi region:

Click to view full size image

There are two obvious emission nebulae here, both typically pink in color. One is seen at one o'clock, centered on hot (circa O9) star Sigma Scorpii. The other emission nebula is larger and fainter, and it is centered on hot (B0) star Tau Scorpii, which is just peeking out in the lower right corner. Most of the other stars here are also surrounded by gas and dust, but they are not hot enough to cause ionization.

Ann

[Skooterp]

Are all the little spots stars in the foreground, or galaxies in the background?
I know the really bright ones are stars within our own galaxy, but I'm not so sure about the little spots that cover the rest of the image.

The glowing hydrogen clouds are known as H II (ionized hydrogen) regions.

I'm not talking about the hydrogen clouds... I'm talking about the small white dots that cover the entire image. Are the stars in the foreground, or galaxies in the background?

[garry]

What exactly is " ionized by energetic starlight"? I mean are the photons that strong? If energetic starlight is this high powered light? If ionized does this mean by radiation, plasma, gas, what? It is not a very god explanation from a scientific viewpoint!

Photons are a fundamental carrier of energy. They are what cause ionization, by knocking electrons off of atoms. Photon radiation is the most common type of radiation- aka light. Plasma and gas can't ionize anything. A plasma is an ionized gas- usually ionized by photons.

Thanks for he explanation, but plasma is ionized by an electric current, (any lab test can demonstrate this), not photons!

[Chris Peterson]

Thanks for he explanation, but plasma is ionized by an electric current, (any lab test can demonstrate this), not photons!

A plasma can be produced in a gas by a variety of energy transfer mechanisms. A plasma is not ionized by an electric current, but may be created by an electric field. Once the plasma is produce by the field, the resulting free electrons will flow, producing a current, but the current itself is a byproduct of the plasma production.

In general, in an astronomical context, plasmas are not produced by electric fields. All the plasmas we observe in nebulas and other regions of diffuse gas are caused by photons exciting the gases.

[jerrythebiker]

Like NoelC, I also wonder what the view of the Milky Way would look like from the LMC. Would a viewer within the LMC see us edge on, tilted, or more face on? How bright would we appear to "them" and how many degrees would we span in "their" sky? Has anyone ever done an artist's rendering of this view? Would love to see one, since I won't be travelling there in the near future.

[Chris Peterson]

Like NoelC, I also wonder what the view of the Milky Way would look like from the LMC. Would a viewer within the LMC see us edge on, tilted, or more face on? How bright would we appear to "them" and how many degrees would we span in "their" sky? Has anyone ever done an artist's rendering of this view? Would love to see one, since I won't be travelling there in the near future.

Well, since the LMC isn't too far off our galactic plane, I guess that the Milky Way from there would look similar to how it looks from here: a faint gray band across much of the sky, brighter near the galactic center. The main difference would be that from the LMC the band would span a large fixed angle, whereas from here it's a ring.

[NoelC]

In general, in an astronomical context, plasmas are not produced by electric fields.

And I would add, in support of Chris' use of the word "usually" (in "A plasma is an ionized gas- usually ionized by photons.") that there's a helluva lot more of that going on out there in the cosmos than there are labs ionizing gas with electric fields.

That said, aren't there powerful electric fields in the Crab Nebula? Or am I remembering magnetic fields?

-Noel

[neufer]

aren't there powerful electric fields in the Crab Nebula? Or am I remembering magnetic fields?

Magnetic fields

There are strong negative feedback effects that currently prevent powerful electric fields from generating naturally.

[Chris Peterson]

That said, aren't there powerful electric fields in the Crab Nebula? Or am I remembering magnetic fields?

There is evidence for regions within the Crab Nebula where there may be strong electric fields- presumably a product of the interaction between the (known) powerful magnetic field and shock fronts of moving particles.

It's a big, complicated Universe, and in regions of high density it's certain that strong electrical fields are present, and probably currents as well (as in stars, for instance). But there is no evidence for significant electrical fields or currents in galaxies, nebulas, or other relatively diffuse regions- certainly not strong enough fields to create ionization and plasmas. All of the nebular plasmas we see are produced by radiation, mainly photons, and any currents flowing in those plasmas are likely to be small.

[kshiarella]

Based on the photograph, the HII regions seem to be concentrated on one side of the Galaxy's central bar, the one with the spiral arm that looks pulled out. Are tidal forces greater on one side, is that side nearer to the Milky Way? It certainly looks like one side of the LMC is seeing more action than the other.

[Ann]

Based on the photograph, the HII regions seem to be concentrated on one side of the Galaxy's central bar, the one with the spiral arm that looks pulled out. Are tidal forces greater on one side, is that side nearer to the Milky Way? It certainly looks like one side of the LMC is seeing more action than the other.

I can't tell you if the "Tarantula side" of the LMC is closer to the Milky Way than the other side, but one thing I know is that it is quite common for galaxies to have most of their star formation in just one side or part of them.

Take a look at galaxy NGC 7678:

Click to view full size image

One arm, to the right of the nucleus, is jam-packed with large emission nebula and large star clusters. The rest of the galaxy is not as active as that. I have a color atlas in UBV, The Color Atlas of Galaxies by James D. Wray, and in that galaxy the bright arm of NGC 7678 shines all blue from ultraviolet radiation, while the rest of the galaxy looks pretty "quiet".

Here is another example of "lopsided star formation", in galaxy NGC 7496. Note the bright white segment of enhanced star formation above and to the right of the nucleus:

Click to view full size image

Ann