xkcd: What If?

[bystander]

Lake Tea

• What if we were to dump all the tea in the world into the Great Lakes?
  How strong, compared to a regular cup of tea, would the lake tea be? — Alex Burman

[bystander]

Pile of Viruses

• What if every virus in the world were collected into one area? How
  much volume would they take up and what would they look like? — Dave

[bystander]

Catch!

• Is there any way to fire a gun so that the bullet flies through the air and
  can then be safely caught by hand? e.g. shooter is at sea level and catcher
  is up a mountain at the extreme range of the gun. — Ed Hui, London

[bystander]

Hitting a comet

• Astrophysicists are always saying things like "This mission to this comet is
  equivalent to throwing a baseball from New York and hitting a particular
  window in San Francisco." Are they really equivalent? — Tom Foster

[bystander]

Star Sand

• If you made a beach using grains the proportionate size of the stars
  in the Milky Way, what would that beach look like? — Jeff Wartes

[Ann]

Very interesting, bystander.

Your post made me think of what stars are resolved in typical Hubble galaxy images. An interesting example is vigorously starforming dwarf galaxy NGC 2414, which has a large underlying population of old mostly reddish stars. This Hubble page allows you to look at NGC 4214 at various resolutions. You can easily spot the young blue giants and supergiants, and you can see the bright orange-red stars that tend to keep the blue supergiants company: these are the red supergiants, the Antareses and Betelgeuses. But much of the field is filled with much fainter red points of light. These are the much more ordinary red giants, the Arcturuses and Dubhes. But the bulk of the galaxy is made up of stars smaller than the Sun, the K- and M-type red dwarfs.

I have to wonder if Hubble has spotted any star in NGC 4214 that is smaller than the Sun.

Ann

[Chris Peterson]

Your post made me think of what stars are resolved in typical Hubble galaxy images.

Me too, although in a slightly different way. Of course, no stars are spatially resolved in HST images, they are only resolved in the sense of being isolated from their backgrounds. The apparent diameter of stars in images is many orders of magnitude larger than their actual diameter. Taking a "typical" star to have a diameter of a million kilometers, and an average distance between stars of a light year, we could say that stars tend to be separated from one another by ten million times their diameter. What the sand analysis fails to point out is that the beach would look very different from any ordinary beach because of this, with sand grains now separated by 10 million millimeters- 10 km between each grain. You'd have to search for years just to find any grains at all!

I have to wonder if Hubble has spotted any star in NGC 4214 that is smaller than the Sun.

Why not? "Resolving" a star in another galaxy has nothing at all to do with the star's size, and is only determined by its brightness and separation from other stars.

[geckzilla]

I think the point was just to have the numbers of stars be represented by the sand, not the distance between them.

[Chris Peterson]

I think the point was just to have the numbers of stars be represented by the sand, not the distance between them.

I agree- that wasn't a criticism of the analysis. Just that Ann's thinking about what it means in terms of seeing stars in other galaxies brings to mind the other component of the analysis, which is the low density of galaxies compared with beaches.

[Ann]

Chris wrote:
"Resolving" a star in another galaxy has nothing at all to do with the star's size, and is only determined by its brightness and separation from other stars.

Small stars typically aren't very bright. Are they bright enough for Hubble to resolve them if they are, say, half as bright as the Sun and located in the disk of a galaxy at the distance of the Virgo Cluster? Let's say that the separation of the stars in this galactic disk is similar to the separation between stars in the Sun's neighborhood.

Ann

[Chris Peterson]

Small stars typically aren't very bright. Are they bright enough for Hubble to resolve them if they are, say, half as bright as the Sun and located in the disk of a galaxy at the distance of the Virgo Cluster? Let's say that the separation of the stars in this galactic disk is similar to the separation between stars in the Sun's neighborhood.

Instrumentally, I think we could detect a star half as bright as the Sun at that distance (I'll do the calculation later). The real problem is separation. Probably, the star would need to be more isolated than is typical.

[geckzilla]

I think Hubble's M82 mosaic is good for getting the feel for resolving different types of stars because it's a smooth haze of stars with plenty of globular clusters to compare the individually resolved stars to because you know more or less what globular clusters are made of. Here is a 12 megabyte JPEG file to pan around in without any sharpening applied so it's easy to tell the difference between a globular and a foreground star... http://www.geckzilla.com/astro/m82_flat.jpg

Unfortunately, the JPEG compression muddles some of it but you can still see clearly that the smaller stars aren't resolved at all but become part of a fine mist or haze. I imagine them like fog particles.

[neufer]

Small stars typically aren't very bright. Are they bright enough for Hubble to resolve them if they are, say, half as bright as the Sun and located in the disk of a galaxy at the distance of the Virgo Cluster?

• 1) The Hubble Space Telescope has located stars with apparent magnitudes of 30 at visible wavelengths.
  
  2) At (the Virgo Cluster's distance of) 16.5 Mpc this corresponds to an absolute magnitude of -1
  
  3) Our own Sun has an absolute magnitude of +4.83
  
  4) If our own Sun was 10 times closer than the Virgo Cluster it still would not be visible in Hubble.

[rstevenson]

... Here is a 12 megabyte JPEG file to pan around in without any sharpening applied so it's easy to tell the difference between a globular and a foreground star... http://www.geckzilla.com/astro/m82_flat.jpg ...

I've never seen such an image before processing, so this is a learning experience for me. Here's a piece of it, for those not able to deal with a 12 MB file, roughly 30% from the left, about in the middle vertically. In this piece I'm sure there are 3 globular clusters. Would all the other spots be stars? And can we say that about a third of them are orange(ish) and perhaps twice that many are blue or white? Or is any apparent colour an illusion at this point in the processing?

And here's a 5X blowup of a bit of that, including the largest of the GCs, so you can see how fuzzy everything is.

m82_5x_globular.jpg (61.54 KiB) Viewed 5804 times

Rob

[geckzilla]

I've never seen such an image before processing, so this is a learning experience for me. Here's a piece of it, for those not able to deal with a 12 MB file, roughly 30% from the left, about in the middle vertically. In this piece I'm sure there are 3 globular clusters. Would all the other spots be stars? And can we say that about a third of them are orange(ish) and perhaps twice that many are blue or white? Or is any apparent colour an illusion at this point in the processing?

Thanks, I was going to put some little close up crops but I was in a rush to go somewhere. I replaced the images you posted with some clearer ones. Yes, the little orange spots are probably big ol' stars or close binary systems or whatever. Panning around you can find a few blue ones, too.

Also, just to clarify it's still processed, there is just no deconvolving/sharpening so it's I guess a little more natural? If you want to view some truly raw data you'd have to view the FITS files themselves.

[rstevenson]

... I replaced the images you posted with some clearer ones. ...

Oops! I forgot my software is set to 80% by default, since that's almost always the best I need.

Rob

[geckzilla]

I mean because yours were second generation JPEGs. I just brought them back up to first generation.

[BMAONE23]

So, If you shrunk the Stars down to the sizes of Sand Grains (and boulders) as appropriate, How far away from Our grain of sand, would Alpha Centauri be?

[Chris Peterson]

So, If you shrunk the Stars down to the sizes of Sand Grains (and boulders) as appropriate, How far away from Our grain of sand, would Alpha Centauri be?

Something like 40 km.

It's easy to forget just how empty space is.

[neufer]

So, If you shrunk the Stars down to the sizes of Sand Grains (and boulders) as appropriate,
How far away from Our grain of sand, would Alpha Centauri be?

About 9.4 kilometres.

[Chris Peterson]

So, If you shrunk the Stars down to the sizes of Sand Grains (and boulders) as appropriate, How far away from Our grain of sand, would Alpha Centauri be?

Something like 40 km.

About 9.4 kilometres.

You want better than that, you're going to have to define what size sand grain represents the Sun.

[geckzilla]

Use the USGS's wonderful chart! How about the medium sized grain of sand right smack in the middle of medium: .354mm

[geckzilla]

Paint the Earth

• Has humanity produced enough paint to cover the entire land area of the Earth? —Josh (Bolton, MA)

[Chris Peterson]

Use the USGS's wonderful chart! How about the medium sized grain of sand right smack in the middle of medium: .354mm

So d = 0.354e-6 km * 4.1e13 km / 1.391e6 km = 10.4 km.

[neufer]

Use the USGS's wonderful chart! How about the medium sized grain of sand right smack in the middle of medium: .354mm

So d = 0.354e-6 km * 4.1e13 km / 1.391e6 km = 10.4 km.

• The distance to that far speck of sand is determined by
  alternately blinking one's eyes separated by 2AU (i.e., ~ 3 inches).