Starship Asterisk*

Anemic Spiral NGC 4921 from Hubble

Explanation: How far away is spiral galaxy NGC 4921? It's surpringly important to know. Although presently estimated to be about 300 million light years distant, a more precise determination could be coupled with its known recession speed to help humanity better calibrate the expansion rate of the entire visible universe. Toward this goal, several images were taken by the Hubble Space Telescope in order to help identify key stellar distance markers known as Cepheid variable stars. Since NGC 4921 is a member of the Coma Cluster of Galaxies, refining its distance would also allow a better distance determination to one of the largest nearby clusters in the local universe. The magnificent spiral NGC 4921 has been informally dubbed anemic because of its low rate of star formation and low surface brightness. Visible in the featured image are, from the center, a bright nucleus, a bright central bar, a prominent ring of dark dust, blue clusters of recently formed stars, several smaller companion galaxies, unrelated galaxies in the far distant universe, and unrelated stars in our Milky Way Galaxy.

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It appears to be common in the coma cluster. Hydrogen is being stripped off by the intergalactic medium, leaving galaxies deficient... this galaxy has a high relative velocity...7,560 km/s, which probably does not help...

Many galaxies in the vicinity look in similar condition.

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I thought I'd seen it all, but ... as has happened multiple times before, this professional astronomer is flummoxed by an APOD! Wow!

I really like today's APOD!


Okay, first off, I want to say that the bluish color of NGC 4921 is not "true color". Since there are such low levels of star formation in NGC 4921, the galaxy is clearly dominated by yellow stars. Nevertheless, the milky bluis-white colors suit this galaxy. After all, you've never heard of a yellow ghost, have you?







Today's APOD reveals a lot of lovely little details. Star formation is heavily concentrated to the gas-rich dust clouds in the galaxy. Strong intergalactic winds sculpt "pillars of creation" in the striking dust lane at right, which almost appears to be on blue fire due to star formation. The bulge of the galaxy looks pale and wan, as if it, too, was a ghost. Diffuse little satellite galaxies can be seen right "above" the galaxy, and at lower right.

NGC 4921 is a handsome ghost of a galaxy.

Ann

I did notice that you can see several distant galaxies through and behind NGC 4921!

How 'bout that pair of Milky Way stars just "below" the center. I wonder if we know whether those are companions or if they just happen to line up closely in our line of sight.

Hmmm. Since this lack of star forming activity likely stems from a loss of free hydrogen, and therefore a relative increase in metallicity, perhaps we should not see it as anemic, but rather as polycythemic.

orin stepanek wrote: ↑Wed May 15, 2019 12:02 pm I did notice that you can see several distant galaxies through and behind NGC 4921!

See an annotated view here.

Chris Peterson wrote: ↑Wed May 15, 2019 2:43 pm
Hmmm. Since this lack of star forming activity likely stems from a loss of free hydrogen, and therefore a relative increase in metallicity, perhaps we should not see it as anemic, but rather as polycythemic.

Not enough plasma

Looks more like a cataract to me.

When was the last time anyone dusted that thing?