Astronomy Q&A game

[Nereid]

Where in the universe?

Is Alpha, Beta, and Maxwell (all three)?
Hint: these three are different from the rest.

Bonus question: why are they different?

 

[Jeebus]

Originally posted by Nereid
Is Alpha, Beta, and Maxwell (all three)?
Hint: these three are different from the rest.

Bonus question: why are they different?

No idea, Nereid, nice question. There not all rays? :) I dunno...

 

[Lonewolf]

Would they be on Venus, and the only features on the planet not named after a female? Maxwell Montes being the only feature named after a male.

 

[Nereid]

Correct Lonewolf!

Want to try to bonus question? To re-state a little more clearly: Why were Alpha Regio, Beta Regio, and Maxwell Montes NOT given female names?

Anyway, it's your turn.

 

[Lonewolf]

Hmm, Alpha Regio and Beta Regio were already there before. Maxwell Montes was named after James Maxwell. I'm not sure were Alpha and Beta weren't changed, or why Maxwell is the only male on a feminine planet. Perhaps you'd like to explain?

Anyway, question time, sticking to a Venus theme. Earth and Venus are very similar in mass and radius, and other features. Why is it then that their atmospheres are dramatically different?

 

[Nereid]

Features on the surface of Venus

Maxwell is the most prominent feature on Venus, as seen in radar. Venus was the first target for radar astronomers (after the Moon), starting in 1961. The Alpha and Beta regions were also observed, and confirmed early.

When the IAU got around to formalising the process of assigning names to solar system objects (1973?), they decided to give only female names to features on Venus (and there are other rules too, see the link). However, they made an exception for Maxwell, Alpha, and Beta, because these were already established.
http://astrogeology.usgs.gov/Projects/PlanetaryMapping/VenusMappers/AppendC.html

 

[Labguy]

Originally posted by Lonewolf
Hmm, Alpha Regio and Beta Regio were already there before. Maxwell Montes was named after James Maxwell. I'm not sure were Alpha and Beta weren't changed, or why Maxwell is the only male on a feminine planet. Perhaps you'd like to explain?

Anyway, question time, sticking to a Venus theme. Earth and Venus are very similar in mass and radius, and other features. Why is it then that their atmospheres are dramatically different?

The lighter gasses (originally) were lost due to higher temperatures because of proximity to the sun. This left the more dense gasses, leading to higher densities and the start of the (increasing) "greenhouse effect". The pressure of Venus' atmosphere at the surface is `90 atmospheres. It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. This dense atmosphere produces a run-away greenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

(Answer stolen from internet, just to get specifics right)...

 

[Lonewolf]

Cool, thanks Nereid. Labguy, you got the question right, so ask away!

 

[Labguy]

Originally posted by Lonewolf
Cool, thanks Nereid. Labguy, you got the question right, so ask away!

Ok, thanks.

Question(s):

(1)What is the name given to very large wisps of dust which cannot be seen in visible light, and are only ~15-30 K?

(2) Where are they found?

(3) What's the energy source of the (very low) tempreature?

 

[Labguy]

Originally posted by Labguy
Ok, thanks.

Question(s):

(1)What is the name given to very large wisps of dust which cannot be seen in visible light, and are only ~15-30 K?

(2) Where are they found?

(3) What's the energy source of the (very low) tempreature?

Ok, guys. You'll find the answer in the same subject/page as the "T-Tauri" answer at the top of this page!

 

[marcus]

Originally posted by Labguy
Ok, thanks.

Question(s):

(1)What is the name given to very large wisps of dust which cannot be seen in visible light, and are only ~15-30 K?

(2) Where are they found?

(3) What's the energy source of the (very low) tempreature?

(1) dust disks
(2) found around stars (e.g. T Tauri stars)
(3) the starlight heats them to the (very low) temperature

 

[Labguy]

Originally posted by marcus
(1) dust disks
(2) found around stars (e.g. T Tauri stars)
(3) the starlight heats them to the (very low) temperature

Close, but only #3 is correct.

 

[marcus]

Originally posted by Labguy
Close, but only #3 is correct.

well, there are "twisters"

?

 

[Nereid]

1) interstellar cirrus
2) just about everywhere, but they're particularly noticable above the galactic plane
3) the ambient radiation, from stars, keeps them warmer than 2.7^oK. As they're more or less transparent, they don't get a chance to cool down below this temperature, and they do absorb starlight, so they are warmer than the CMB.

They were first found (convincingly) by IRAS, and caused astronomers to go back and re-do an awful lot of 'absorption' calculations; previously interstellar absortion was assumed to be more or less uniform; the cirrus showed that is was highly irregular and patchy.

 

[Labguy]

Originally posted by Nereid
1) interstellar cirrus
2) just about everywhere, but they're particularly noticable above the galactic plane
3) the ambient radiation, from stars, keeps them warmer than 2.7^oK. As they're more or less transparent, they don't get a chance to cool down below this temperature, and they do absorb starlight, so they are warmer than the CMB.

They were first found (convincingly) by IRAS, and caused astronomers to go back and re-do an awful lot of 'absorption' calculations; previously interstellar absortion was assumed to be more or less uniform; the cirrus showed that is was highly irregular and patchy.

That's right on all three points. It was an IRAS site I found this on, several years ago.

That makes it your question!

 

[Nereid]

missing mass

We've all read about the 'missing mass' or 'dark matter' which is supposed to exist in the galactic halo and bulge.

Red dwarf stars have been observed for a long time (the closest star - apart from the Sun - to us is a red dwarf); more recently even fainter stars - brown dwarfs - have been discovered. There must also be even smaller objects alone out in interstellar space, formed independently or ejected from a nascent planetary system.

How can we be so sure that these isolated faint objects - and others such as cold white dwarfs, neutron stars, even stellar-mass black holes - cannot comprise more than a small fraction of the halo or bulge missing mass?

 

[Labguy]

Originally posted by Nereid
We've all read about the 'missing mass' or 'dark matter' which is supposed to exist in the galactic halo and bulge.

Red dwarf stars have been observed for a long time (the closest star - apart from the Sun - to us is a red dwarf); more recently even fainter stars - brown dwarfs - have been discovered. There must also be even smaller objects alone out in interstellar space, formed independently or ejected from a nascent planetary system.

How can we be so sure that these isolated faint objects - and others such as cold white dwarfs, neutron stars, even stellar-mass black holes - cannot comprise more than a small fraction of the halo or bulge missing mass?

I would have to guess that you are looking for "galactic rotation rates", without a website to quote.

If you limit your question as asked, galactic bulge and halo, then any more significant (missing) mass in the bulge would cause stars, and spiral arms such as ours, to rotate faster than observed and faster than the "known mass" formulae would predict. If the halo had significantly more (missing) mass, and the bulge remained as observed, then (1) rotation would be slower than observed and (2) there would be more unstability and disruption in the disk-shaped spiral arms, in the case of a spiral galaxy.

In Elliptical galaxies the same would be true for rotation rates, without a central "bulge". As of now, the only increased rotation rates seen, in most galaxies, is the rapid increase near the cores indicating a massive BH in the center. M-87 is calculated to have a Billion Ms BH in the core, ours (Milky Way Galaxy) is ~1 million Ms.

 

[Nereid]

Only the Milky Way

These are all good points Labguy.

My fault for not asking the question sufficiently clearly.

First, I am only asking about the Milky Way halo and bulge, not other galaxies.

Next, you're right; it is the rotation curves which provide clear indications that the amount of mass is greater than what can be observed as luminous stars, gas, and dust.

What is the missing mass? There are good reasons for thinking it cannot be faint, unobserved objects such as (distant) red dwarfs, brown dwarfs, isolated planets, etc. I'm not counting rocks, pebbles, even smallish asteroid-sized bodies in this; nor 'big dust'.

My question is: why are we confident that these faint objects (brown dwarfs etc) cannot comprise more than a small fraction of the missing mass?

 

[Labguy]

Brown Dwarfs; From a stellar formation graph and formula page:

These so- called "aborted stars" have a mass around .08Ms, and can't convert hydrogen into helium (Their cores never reach the threshold temperature for hydrogen burning.). The only energy radiated is due to gravitational contraction (Kelvin-Helmholtz contraction), which is why they are difficult to detect unless they are located near us. To determine if brown dwarfs contribute substantially to the dark matter mass, we need to estimate their number. We can use the Stellar Mass Function, F(M) (proportional to M-2.33), to extrapolate the number of brown dwarfs from the numbers of more massive stars. If F(M)dM = the number of stars with mass between M and M+dM, then the total mass contribution from brown dwarfs can be written as (M)(F(M))dM. The question is, how does F(M) behave for very low M? F(M) is proportional to M-2.33 only for main sequence stars, not for brown dwarfs. In general, giant clouds of gas and dust collapse, then fragment. The smaller fragments, which become K and M stars, are more abundant then the larger fragments, which become O and B stars. However, the amount of small fragments drops off right near the size needed to make brown dwarfs, limiting the number of brown dwarfs that could exist, and also providing an uncertainty as to that number.

If all the dark matter is composed of brown dwarfs, we would need one brown dwarf every 30 cubic ly of space, many trillions over the entire Milky Way. The number of known brown dwarfs and brown dwarf candidates in our section of the galaxy are exceedingly slim, however, making it unlikely that the density of the galaxy's brown dwarf population accounts for significant dark matter.

(Some original, some stolen text)

 

[Nereid]

Extrapolating the SMF suggests that there would be too few such low mass objects.

What observations have been made that show there are insufficient non-luminous (or low-luminosity) massive objects to account for the missing mass?

 

[Labguy]

Originally posted by Nereid
Extrapolating the SMF suggests that there would be too few such low mass objects.

What observations have been made that show there are insufficient non-luminous (or low-luminosity) massive objects to account for the missing mass?

Is this an additional question, or required as an answer to your last question? It wasn't specified.

 

[marcus]

Here is the original question:

Originally posted by Nereid

...How can we be so sure that these isolated faint objects - and others such as cold white dwarfs, neutron stars, even stellar-mass black holes - cannot comprise more than a small fraction of the halo or bulge missing mass?

"be so sure" suggests that N. is looking for observational evidence, does it not? The model of stellar formation predicts mass distributions, but is a model---it might even have to be modified some day, who knows? Here is the same question rephrased:

Originally posted by Nereid

...What observations have been made that show there are insufficient non-luminous (or low-luminosity) massive objects to account for the missing mass?

This is a really interesting question which I don't think has been addressed yet in this thread. How come we're so sure---on the grounds of what observational evidence? How did we "see" the absence of billions of invisible small-size stars? Someone knows the answer here, I expect (besides Nereid, I mean). Let's hear from some of you others!

 

[Labguy]

Originally posted by marcus
Here is the original question:



"be so sure" suggests that N. is looking for observational evidence, does it not? The model of stellar formation predicts mass distributions, but is a model---it might even have to be modified some day, who knows? Here is the same question rephrased:



This is a really interesting question which I don't think has been addressed yet in this thread. How come we're so sure---on the grounds of what observational evidence? How did we "see" the absence of billions of invisible small-size stars? Someone knows the answer here, I expect (besides Nereid, I mean). Let's hear from some of you others!

Ok, then someone else can answer what Nereid "might have thought" instead of answering the question as I did.

I think questions, like my last 3-parter, should all be asked at one time instead of "comebacks" saying that "you're right, but now add more". I can't guess ahead as to what someone "might have meant", just answer the question as asked.

 

[Nereid]

The original question again

How can we be so sure that these isolated faint objects - and others such as cold white dwarfs, neutron stars, even stellar-mass black holes - cannot comprise more than a small fraction of the [clarification: Milky Way] halo or bulge missing mass?

Galaxy rotation curves - including those of the Milky Way - are strong observational evidence that there is missing mass; they don't give any clues as to what it is, or is not.

Extrapolation of the SMF suggests that lower mass compact objects may not make up very much of the missing mass; it's not enough to give much confidence (as marcus correctly noted). It also doesn't address "cold white dwarfs, neutron stars, even stellar-mass black holes"

Hint: evidence for the existence of (non- or sub-)luminous massive, compact, halo objects

 

[Labguy]

Originally posted by Nereid
Galaxy rotation curves - including those of the Milky Way - are strong observational evidence that there is missing mass; they don't give any clues as to what it is, or is not.

Extrapolation of the SMF suggests that lower mass compact objects may not make up very much of the missing mass; it's not enough to give much confidence (as marcus correctly noted). It also doesn't address "cold white dwarfs, neutron stars, even stellar-mass black holes"

Hint: evidence for the existence of (non- or sub-)luminous massive, compact, halo objects

Originally posted by nightbat on the stellar astrophysics page; many observations:

Here are a few:

http://spaceflightnow.com/news/n0308/11halo/
http://arxiv.org/abs/astro-ph/0206126
http://aanda.u-strasbg.fr:2002/arti...092/aa1092.html
http://www.strw.leidenuniv.nl/information/normgal.html
http://xxx.lanl.gov/abs/astro-ph/9501068
http://www.star.bris.ac.uk/nam/abs_galaxy_dynamics.html
http://www.blackwell-synergy.com/li...03.06680.x/abs/

You will find many more on the internet.

If that's enough, give the credit to nightbat, and notify him that he answered the question here. Emphasis on "You will find many more on the internet."

 

[marcus]

Originally posted by Nereid
We've all read about the 'missing mass' or 'dark matter' which is supposed to exist in the galactic halo and bulge.

Red dwarf stars have been observed for a long time (the closest star - apart from the Sun - to us is a red dwarf); more recently even fainter stars - brown dwarfs - have been discovered. There must also be even smaller objects alone out in interstellar space, formed independently or ejected from a nascent planetary system.

How can we be so sure that these isolated faint objects - and others such as cold white dwarfs, neutron stars, even stellar-mass black holes - cannot comprise more than a small fraction of the halo or bulge missing mass?

I'll hazard a guess: by "microlensing" observations----which yield an upper bound on the abundance of massive dark objects

the original question is pretty clear---we take for granted that much of the Milkyway mass is dark in the sense that we can't detect radiation from it-----either it is stuff that shines too faintly to detect or that doesn't shine at all. That is not at issue here.

The question is, how do we know (based on what observations do we know) that this invisible mass does not consist of objects in the large-planet-to-small-star range?

The stellar-mass-distribution function is just an empirically arrived at thing with limited predictive power so we can't use it to exclude the possibility that at some time in Milkyway history a lot of un-shining massive objects formed (Nereid mentioned stellar-mass black holes, cold white dwarfs etc)

But people can actually see invisible massive objects by their gravitational-lens effect as they pass in the foreground of visible objects----microlensing. So they have been able to estimate how many dark massive objects there are.

 

[Nereid]

MACHO

Yep, marcus is right.

Nearly a decade ago now several long term research projects got underway to detect gravitational lensing of background stars. They had several objectives, one of which was to estimate the space density of MAC(H)Os - massive compact (halo) objects. As one of these passes between the line of sight between us and a distant star, it lenses the star's light, and we see a characteristic brightening then fading.

Although it's no longer maintained, the MACHO consortium's homepage (http://wwwmacho.mcmaster.ca/) has good background (including a photo of the Mt Stromlo observatory, now destroyed ). To read a particular paper - from the publications list - copy and paste the title into Google; to look up the results from other lensing projecs (e.g. OGLE), type the project name into Google.

There have also been dedicated HST searches for red dwarfs in the halo - far fewer were found than would be needed to comprise a significant fraction of the missing mass.

(you can do a simple calculation for yourself - given proxima centuri, distance ~ 4 ly, mag ~11, the nearest red dwarf - how far away would a similar red dwarf be if its observed mag were 25?)

Your turn marcus.

 

[marcus]

Originally posted by Nereid

Your turn marcus.

Sure. What is the name of Siobahn Morgan's dog?


[prof Morgan was the first person I know of to post an online
cosmology calculator that actually gives the recession speed
of an object of known redshift at the time it emitted the light
we are receiving from it.

http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

be sure to put in 0.27 for matter density and 0.73 for Lambda or cosmological constant, then it will compute recession speeds and distances from whatever redshift you give it

Siobahn's homepage is
http://www.earth.uni.edu/smm.html ]

For extra credit, name 3 of Siobahn's favorite superhero comic book characters.

For optional, entirely voluntary additional extra credit: a quasar has been observed at redshift 6.4---with what speed was the quasar receding from us when it emitted the light?

 

[Labguy]

Originally posted by marcus
Sure. What is the name of Siobahn Morgan's dog?


[prof Morgan was the first person I know of to post an online
cosmology calculator that actually gives the recession speed
of an object of known redshift at the time it emitted the light
we are receiving from it.

http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

be sure to put in 0.27 for matter density and 0.73 for Lambda or cosmological constant, then it will compute recession speeds and distances from whatever redshift you give it

Siobahn's homepage is
http://www.earth.uni.edu/smm.html ]

For extra credit, name 3 of Siobahn's favorite superhero comic book characters.

For optional, entirely voluntary additional extra credit: a quasar has been observed at redshift 6.4---with what speed was the quasar receding from us when it emitted the light?

Millie

Batman
Green Lantern
Aquaman

I have no "direct observations" to back this up.

 

[marcus]

Originally posted by Labguy
Millie

Batman
Green Lantern
Aquaman

I have no "direct observations" to back this up.

Your go, Labguy, right on all counts

I see you did not opt for the additional extra credit---a redshift 6.4 quasar was receding at 3 times c when it emitted the light we receive from it----or more accurately, by Siobahn's calculator, 2.88 c.