Starship Asterisk*

Nearby Spiral Galaxy NGC 4945

Explanation: Large spiral galaxy NGC 4945 is seen edge-on near the center of this cosmic galaxy portrait. In fact, it's almost the size of our Milky Way Galaxy. NGC 4945's own dusty disk, young blue star clusters, and pink star forming regions standout in the sharp, colorful telescopic image. About 13 million light-years distant toward the expansive southern constellation Centaurus, NGC 4945 is only about six times farther away than Andromeda, the nearest large spiral galaxy to the Milky Way. Though this galaxy's central region is largely hidden from view for optical telescopes, X-ray and infrared observations indicate significant high energy emission and star formation in the core of NGC 4945. Its obscured but active nucleus qualifies the gorgeous island universe as a Seyfert galaxy and home to a central supermassive black hole.

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Looks to have a lot of dust! I imagine a lot of star forming in this galaxy!

orin stepanek wrote: ↑Thu Aug 22, 2019 10:53 am Looks to have a lot of dust!

Indeed so! In fact, I can't remember ever seeing an APOD of a galaxy so chock full of dust.

I imagine a lot of star forming in this galaxy!

Well, at least we can say for sure that there WAS a lot of star forming in this galaxy's past, as dust is the accumulated ash (metals) of generations of stars that have come and gone. But stars need gas (mostly hydrogen) to form and run. Could this galaxy be depleted in gas since it has already converted so much of its fuel into metals?

neufer wrote: ↑Thu Aug 22, 2019 11:43 am

Click to play embedded YouTube video.

Nice find art! A song for today's APOD!

BDanielMayfield wrote: ↑Thu Aug 22, 2019 11:29 am

orin stepanek wrote: ↑Thu Aug 22, 2019 10:53 am Looks to have a lot of dust!

Indeed so! In fact, I can't remember ever seeing an APOD of a galaxy so chock full of dust.

I imagine a lot of star forming in this galaxy!

Well, at least we can say for sure that there WAS a lot of star forming in this galaxy's past, as dust is the accumulated ash (metals) of generations of stars that have come and gone. But stars need gas (mostly hydrogen) to form and run. Could this galaxy be depleted in gas since it has already converted so much of its fuel into metals?

Indeed, this galaxy seems to be churning and choking with dust! But while it contains huge amounts of dust itself, it is also reddened by dust in our own galaxy.






















It is interesting to compare NGC 4945 with IC 342, which is called "The Invisible Galaxy" because it is hiding behind so much Milky Way dust! NGC 4945 is also reddened by foreground Milky Way dust, but it is choking in its own dust, too. Note that both these galaxies are yellowish in color (because of dust reddening), but both of them show off red emission nebulas ionized by young blue stars. But the blue light from the young stars is scattered away by all the dust.

But hey, look at the similarities between these two galaxies!

Total B magnitude, NGC 4945: 9.27.
Total B magnitude, IC 342: 9.21.

Effective B-V index, NGC 4945: 1.36 (that's very red).
Effective B-V index, IC 342: 1.10 (also red, but not quite so red).

B magnitude minus far infrared magnitude, NGC 4945: 2.58 (that's very infrared and dusty).
B magnitude minus far infrared magnitude, IC 342: 2.26 (that's also infrared and dusty).

Moreover, Principal Galaxy Catalog claimed that the optical galactic luminosity of IC 342 is the same as the optical luminosity of the Milky Way, and today's APOD caption claimed that NGC 4549 is almost the same size as the Milky Way!

Finally, consider the weird shape of NGC 4945. You know what its shape looks like, don't you?

orin stepanek wrote: ↑Thu Aug 22, 2019 10:53 am Looks to have a lot of dust! I imagine a lot of star forming in this galaxy!

What happens when stars "try" to form from hydrogen that has lots of other elements in it? Failed star? Could a star form with a solid (or liquid) core made from other elements?

FLPhotoCatcher wrote: ↑Thu Aug 22, 2019 7:48 pm

orin stepanek wrote: ↑Thu Aug 22, 2019 10:53 am Looks to have a lot of dust! I imagine a lot of star forming in this galaxy! 8-)

What happens when stars "try" to form from hydrogen that has lots of other elements in it? Failed star? Could a star form with a solid (or liquid) core made from other elements?

Even in these very dusty places, elements other than hydrogen only represent a percent or two of the total mass. So stars that form in such areas, while considered "metal rich", are still mostly hydrogen. And when they start forming, they tend to blow away a lot of dust, preventing it from accumulating in the star.

Chris Peterson wrote: ↑Thu Aug 22, 2019 9:11 pm

FLPhotoCatcher wrote: ↑Thu Aug 22, 2019 7:48 pm

orin stepanek wrote: ↑Thu Aug 22, 2019 10:53 am Looks to have a lot of dust! I imagine a lot of star forming in this galaxy!

What happens when stars "try" to form from hydrogen that has lots of other elements in it? Failed star? Could a star form with a solid (or liquid) core made from other elements?

Even in these very dusty places, elements other than hydrogen only represent a percent or two of the total mass. So stars that form in such areas, while considered "metal rich", are still mostly hydrogen. And when they start forming, they tend to blow away a lot of dust, preventing it from accumulating in the star.

Wouldn't the lighter elements like hydrogen be blown away a lot more than the heavier elements?

FLPhotoCatcher wrote: ↑Thu Aug 22, 2019 9:26 pm

Chris Peterson wrote: ↑Thu Aug 22, 2019 9:11 pm

FLPhotoCatcher wrote: ↑Thu Aug 22, 2019 7:48 pm

What happens when stars "try" to form from hydrogen that has lots of other elements in it? Failed star? Could a star form with a solid (or liquid) core made from other elements?

Even in these very dusty places, elements other than hydrogen only represent a percent or two of the total mass. So stars that form in such areas, while considered "metal rich", are still mostly hydrogen. And when they start forming, they tend to blow away a lot of dust, preventing it from accumulating in the star.

Wouldn't the lighter elements like hydrogen be blown away a lot more than the heavier elements?

With 95% of the mass being hydrogen, that's what forms the star. It doesn't start blowing until it has started fusing, or is very close to it. And yes, once that happens, it clears out a bubble around it- hydrogen, other gases, and dust alike.

Chris Peterson wrote: ↑Thu Aug 22, 2019 9:48 pm

FLPhotoCatcher wrote: ↑Thu Aug 22, 2019 9:26 pm

Chris Peterson wrote: ↑Thu Aug 22, 2019 9:11 pm

Even in these very dusty places, elements other than hydrogen only represent a percent or two of the total mass. So stars that form in such areas, while considered "metal rich", are still mostly hydrogen. And when they start forming, they tend to blow away a lot of dust, preventing it from accumulating in the star.

Wouldn't the lighter elements like hydrogen be blown away a lot more than the heavier elements?

With 95% of the mass being hydrogen, that's what forms the star. It doesn't start blowing until it has started fusing, or is very close to it. And yes, once that happens, it clears out a bubble around it- hydrogen, other gases, and dust alike.

Am wondering what you meant by, "when they start forming, they tend to blow away a lot of dust, preventing it from accumulating in the star."

FLPhotoCatcher wrote: ↑Fri Aug 23, 2019 6:32 am

Chris Peterson wrote: ↑Thu Aug 22, 2019 9:48 pm

FLPhotoCatcher wrote: ↑Thu Aug 22, 2019 9:26 pm

Wouldn't the lighter elements like hydrogen be blown away a lot more than the heavier elements?

With 95% of the mass being hydrogen, that's what forms the star. It doesn't start blowing until it has started fusing, or is very close to it. And yes, once that happens, it clears out a bubble around it- hydrogen, other gases, and dust alike.

Am wondering what you meant by, "when they start forming, they tend to blow away a lot of dust, preventing it from accumulating in the star."

Good question, FLPhotoCatcher. I recommend this site:

Stars and Solar Physics wrote:

Astronomers have struggled to understand how the largest stars — up to 120 times as massive as the Sun — can form by sucking in nearby matter. The problem is that, once a star reaches about 20 solar masses, the outward force of its intense radiation exceeds the gravitational force that pulls in matter.

Now, however, Mark Krumholz of the University of California, Santa Cruz, and colleagues, have shown that radiation pressure should not halt accretion. To do this they carried out a three-dimensional computer simulation of massive star formation using their ORION programme (Sciencexpress.org). This involved modelling a gas cloud of 100 solar masses and then watching it evolve over the equivalent of several tens of thousands of years. They found that a central protostar formed after 3600 years and that the star continued to accrete material unimpeded by radiation pressure for 20,000 years after that.

It was at this point that the simulated star became large enough for its radiation pressure force to exceed its gravitational force. Because the radiation is hottest closest to the star, gas nearer the star feels a greater push than gas further away. Gas from further away therefore falls towards the star but piles up where the pressure is greatest, causing it to form two bubbles of gas (on opposite sides of the star) with only radiation inside.

Krumholz’s team found that these bubbles become clumpy, meaning that in some places radiation blows out sections of the bubble wall, whereas in other places dense filaments of gas form though the bubble, allowing gas to be transported to the star.

Ann

FLPhotoCatcher wrote: ↑Fri Aug 23, 2019 6:32 am

Chris Peterson wrote: ↑Thu Aug 22, 2019 9:48 pm

FLPhotoCatcher wrote: ↑Thu Aug 22, 2019 9:26 pm

Wouldn't the lighter elements like hydrogen be blown away a lot more than the heavier elements?

With 95% of the mass being hydrogen, that's what forms the star. It doesn't start blowing until it has started fusing, or is very close to it. And yes, once that happens, it clears out a bubble around it- hydrogen, other gases, and dust alike.

Am wondering what you meant by, "when they start forming, they tend to blow away a lot of dust, preventing it from accumulating in the star."

I simply meant that there's a self-limiting process during star formation that prevents a star from growing once it start fusing. At that point it clears out its environment and no more material contributes to the star. The winds that clear out that bubble move dust away just as effectively as gas. There's no period where dust is still available to contribute mass but gas is not.