PlutonianEmpire's Astronomy Questions Thread

Straight-forward question, I guess.

Assuming standard conditions of 1 bar atmosphere pressure at the surface on a habitable planet, on a clear day at observer location, does the spectral type/temperature of a star influence how it appears to a human observer during sunsets (eg, cooler sun appears redder)?

I would assume so. Rayleigh scattering preferentially scatters shorter wavelengths of course. I don't know how noticeable changes in the star's temperature would be at sunset though. From what I've been able to figure out, until stars get down to below 3000 K, they tend to look white to the eye.

Lazarus?

I've been meaning to work up some code that would let me play around with this kind of thing for some time. Never got round to it though.

Planetary Habitability Laboratory give us http://phl.upr.edu/library/media/sunsetofthehabitableworlds - not sure how accurate that is though.

I think that picture is supposed to depict what each star's angular size would be...although some are redder than others.

-M-

I recently remembered an old Star Trek (TNG) episode featuring two gas giants colliding and forming a new star.

How feasible, or likely, might two high mass brown dwarfs, say 70 M_j each, collide? And would it end up with a new star if the speeds and angles were favorable, in the real Universe? I imagine it'd become a single M dwarf.

Also, is it possible for a brown dwarf to achieve such a high mass if it formed through core accretion? If so, if both bd's in the collision question formed this way, what would happen to the original cores? I imagine the cores (before the collision) would be of an entirely different state of matter than that of a Jovian's core?

I don't know why it wouldn't be possible.
I would think that a 70 M_J brown dwarf, if formed through core accretion, would have a solid core. But I suspect that it would be rare for such a massive object to form that way.

Mmmmm... hydrogen-burning planets. That would certainly mess things up.

Where does the brown dwarf desert lie for A- and B-type stars?

Coming back to this one...

As far as I can tell, all hydrogen-burning stars are hot enough to be "white hot", before atmospheric filtering, as far as I can tell this should be true even for the earlier L-dwarfs. Should be able to see colours without problem (it would not be the usual gloomy orange-red depiction) - with the star high in the sky, the sky would still appear blue, even though the chromaticity would be very different. On the other hand, the stellar disc would not be as bright. So it would not surprise me if the effect of the greater atmospheric absorption at sunset has a more dramatic effect.

I may be wrong about all this though.

Are there any tables or lists similar to this: http://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_notable_celestial_objects

That lists Earth-based sources of light for easier comparisons, since some folks, such as myself, have terrible visual imagination skills? I mean, how do I visualize or imagine in my head what the Sun looks like from Neptune with nothing to compare it with?

If you're not afraid of doing some math, there are 2 formulas for this. One is for calculating the apparent magnitude and one is for calculating the angular diameter. With these two you'll get a good idea of what X looks like from Y.
I don't have these formulas with me right now, when I find them I'll tell you.

Not what I'm looking or asking for. Sorry.

I came up with something for folks on another site, though it is on the 'rough side'. I think of it as the 'light bulb comparison' :

F5V star - average absolute magnitude 3.5 - halogen work light
G2V star - average absolute magnitude 4.7 - 100 watt yellow light bulb.
G9V star - average absolute magnitude 5.7 - 40 watt light bulb
K4V star - average absolute magnitude 6.5 - large size orange Christmas tree bulb
K8V star - average absolute magnitude 8 - small size orange Christmas tree bulb (icicle light)
M1-2V star - average absolute magnitude 10 - small red LED light of sort found on modems and other appliances.

For giant stars, very roughly, a bank of floodlights, of the sort found in sports stadiums.
For the fainter M dwarfs, glow in the dark stickers.

Again, this is a rough comparison.

Short of "shining a laser in your eye," I don't think there a lot of terrestrial examples of light sources that we typically come across that compare to how bright the sun is from somewhere in the Solar system.

I know wolfram alpha displayed a distance from a 100 watt light bulb as comparison, but since they now hide behind a paywall, and since incandescent bulbs are now officially banned here (from mfg. and imports, anyway), I figured posting this question would reveal alternative x distance to y light of z brightness/luminosity.

A 100 watt light bulb is 100 watts, regardless of type.

The difference between magnitudes is a hair over 2.5.  Hence, a 4.5 magnitude star is about 2.5 times brighter than a 5.5 magnitude star.

You posted a list giving a great many apparent magnitude values.  Pick one of those values as the '100 watt light bulb' and scale the rest accordingly.

Your last post sounds like you are trying to do photometric distances.  For stars:

1) Assume a given absolute magnitude based on spectral type.  You will need a chart for this, if you are interested, I'll post one.

2) Subtract absolute magnitude from Visual or apparent magnitude to get a 'Distance Modulus' (DM).

3) Distance Modulus to distance = 10 ^ (0.2*(DM+5)).  That gives the distance to the star in parsecs.

Word of warning: I spent a great deal of time at 'Vizier' plowing through one photometric distance catalog after another and comparing those distances with later parallax (usually Hip distances).  About half the time, photometric distances will be off by more than 20%.  

I did develop a somewhat more complex photometric system that is good to within 20% about 75 - 80% of the time, which is about as good as it gets.

ThinkerX wrote:

Your last post sounds like you are trying to do photometric distances.

You are correct. I made a fictitious trinary system in Celestia, in which the tertiary companion was on a very long, epochal orbit; perihelion of over 11 AU, and aphelion of over 8,200 AU; and was wondering how to visualize each other's apparent magnitudes on their respective hypothetical planets as the stars progressed in their orbits.

Quote :

You are correct. I made a fictitious trinary system in Celestia, in which the tertiary companion was on a very long, epochal orbit; perihelion of over 11 AU, and aphelion of over 8,200 AU; and was wondering how to visualize each other's apparent magnitudes on their respective hypothetical planets as the stars progressed in their orbits.

So...all the stars are 'in system'. Presumably you know the absolute magnitudes of each star type, including the tertiary, as well as its distance at various points. What you are looking for, then, is a means to determine the 'apparent magnitude' from this, and some sort of standard to compare it with.

Yep. Since Celestia already calculates the apparent visual magnitudes at various distances, I was indeed looking for something for easier comparisons. :-)

Magnitude may not be the best way to figure out what it would look like.

Remember the surface brightness does NOT fall off with distance.

Light falls off as 1/r^2
Angular size falls off as 1/r^2

The two cancel out.

This holds until the apparent size gets sufficiently low that you have to start considering diffraction.

20/20 vision implies being able to resolve a separation of 1 minute of arc, the distance at which the Sun's apparent diameter is 1 minute of arc is about 32 AU, just beyond the orbit of Neptune.

Hi, can someone explain radial velocity to me? I ask because if I understand it correctly, it measures the minute variations in distance from the observer. Yet, i frequently hear that term being used to describe the measurements of how much an object wobbles from side to side (left or right or up or down). Is that contradictory, or do they both accurately describe radial velocity measurements?

Radial velocity is the component of the star's velocity toward or away from the observer. The side-to-side or up-down component is the "tangental velocity" and not measurable spectroscopically.

If you're familiar with redshifts and blueshifts, then you've pretty much got the idea of Doppler spectroscopy down. A spectrum from a source is red-shifted (wavelengths get longer) if the source is moving away, and blueshifted (wavelengths made shorter) if the source is approaching us.

The offset between a spectral line in a star's spectrum and that same spectral line in a reference spectrum tells you the radial velocity of the star at the time of measurement.

Aha, that makes much more sense, especially since I'm already familiar with redshifts and blueshifts.

And I believe the same is true when using RV for planet-hunting?

Exactly. Watching the red-shifting and blue-shifting of the spectrum of the star gives you the radial velocity of the star, since it's motion toward or away from you causes that shifting of the spectrum via the Doppler effect.

This cute animation illustrates the process with a double-line spectroscopic binary. Here, an absorption line from both bodies are visible, though in the case of a star+planet system, the planet is usually too dim to produce a noticeable spectrum and all you're left with is the star's spectrum shifting back and forth.