[Martin]

this thread can be for stashing links to webpages with good explanations of astronomy stuff

in Cosmology forum I just saw where aman asked about the slingshot effect (used a lot to save fuel on missions to the outer planets)
and swansont gave this link:
http://www.mathpages...me/kmath114.htm

explaining clearly how the slingshot maneuver gains energy and
angular momentum (taking away from the planet being used)
and then Jenab confirmed having seen slingshotting in simulations
he'd run

http://www.sciencefo...30823#post30823

I'm thinking of adding other good links i see to this thread, to have them handy. like link-answers to astronomy FAQ. Join in if you feel like it.

[Radical Edward]

I like this idea. so much so that I am going to sticky it :)

[Martin]

Radical Edward said:

I like this idea. so much so that I am going to sticky it :)

Great! I hope other people will add some neat links. I'll post a few I have here too.

here are a couple of goodies:
Ned Wright's cosmology website and FAQ

http://www.astro.ucl...ht/cosmolog.htm

http://www.astro.ucl...mology_faq.html

Wendy Freedman and Michael Turner's "Measuring and Understanding
the Universe"
http://arxiv.org/astro-ph/0308418


a lot of good astronomy links are graphic rather than verbal, such as
images from the HST and computer animations, also Ned Wright has a calculator that lets you calculate from something's redshift how far away it is.

I'm interested to see what links others here have found useful so I wont rush to post a lot of my favorites

[Martin]

there are a lot of great astronomy links, which other SFN posters have used some, or I have seen other places. I hope they get added.

In the meantime this thread could be a collection site for other useful astronomy/astrophysics stuff. Like a copy/pasteable version of the
Friedmann equations, now that we have Latex





this is with c = 1 units, which simplifies things some.
the scale factor of the metric (whose increase is the expansion of the universe) is denoted by the letter a.
k is a spatial curvature parameter used to distinguish three cases
k = -1, 0, +1 for negative curvature, spatially flat, positive curvature

rho is an energy density, and easy to confuse with p pressure

the universe appears to be spatially flat, the critical density rho_crit is that needed for it to be perfectly flat with k = 0

the Hubble parameter H is defined to be the time derivative a' of the scale parameter a, divided by a.

In the critical density case of a spatially flat universe the first Friedmann equation boils down to



algebraically that turns into the formula for the critical density

[Martin]

Here are two good online cosmology calculators

Siobahn Morgan's
http://www.earth.uni...ogy/cosmos.html

and Ned Wright's
http://www.astro.ucl.../CosmoCalc.html

homepages for Morgan
http://www.earth.uni.edu/smm.html
and Wright
http://www.astro.ucl...ight/intro.html

To use Siobahn's calculator put Lambda = 0.73
Omega = 0.27
H = 71 (or leave her default value of H = 70, nearly the same)
those are the dark energy and the matter densities as fractions of rho crit,
and H is the present value of the Hubble parameter

then put in any redshift z,
like z =1 or 3 or 10 and it will tell you how far away the thing
was when it emitted the light we are now getting from it
and how far away it is now
and how fast it was receding then
and how fast it is receding now, at the present moment

[Martin]

New paper of Edward Witten in latest issue of Nature
link to online copy (for subscribers) is in the 3 June post of Woit's blog
http://www.math.colu...edu/~woit/blog/
paper involves dark energy (which is an astronomy/cosmology topic!)
and concerns dark energy, the Higgs mass, and electroweak
symmetry breaking

[admiral_ju00]

All you ever want to know about Nebulas(have some more links, but too lazy at the moment to go digging for them)

http://astronomynote.../evolutn/s1.htm
http://blackskies.com/neb101.htm
http://observe.arc.n...h_contents.html

[Martin]

njnightsky][url]www.njnightsky.com[/url said:

is my favorite of course it's mine. :D

thanks to admiral and to jim for adding good links to this sticky
hope more do
jim's link is a good way to get the latest news, I noticed 3 items from today 29 June including a nasa picture of NGC 7331
some 50 million LY away in Pegasus
a galaxy some call a "twin" of the milkyway
"... Since we're inside our galaxy, many of its interesting features are shrouded by dust, so looking at NGC 7331 is like looking into a mirror 50 million light-years away...."

congratulations on having that site. do you update the main page fairly regularly so it really is fresh news, as I experienced just now, that would be great!

[Martin]

Jim, if not too much trouble could you post a link here (in this sticky thread) to a star map
or a set of star maps.
the idea being if someone shows up at SFN asking something like
where's Arcturus?
where and when can I see the constellation Pegasus?
(not those particular questions but ones like them)
then we have the link to give them, for star maps

[Nalos Surith]

Don't forget http://space.com and http://www.nasa.gov :D

[Martin]

Radical Edward said:

I like this idea. so much so that I am going to sticky it :)

this sticky seems to be growing! more people are adding astronomy links!
thanks to admiral, nightsky, and nalos

I just thought of a good link which is a starmap with the Microwave Background dipole temperature variation superimposed

http://aether.lbl.gov/www/projects/u2/

it shows there is a doppler hotspot in the direction of Leo
because we are heading in that direction at some speed like
1.23 thousandths of the speed of light---in absolute space terms

I think it is a really cool map and rather old----the result was reconfirmed by satellite observatory in the 1990s--- but the original result, gotten by U2 plane flying around measuring the microwave temp in various directions, turned out quite accurate.


-----------------------------

[Martin]

If anybody here at SFN ever wants to convert between ordinary sky coords and galactic coordinates there is an online calculator that I've sometimes used---if it is still up and running:

Professor Murphy's online calculator.
(Johns Hopkins)
http://fuse.pha.jhu....ls/eqtogal.html

for example, the hotspot in Leo could be expressed in coordinates in two different ways

ordinary:(11 h 12 m, -7.22 degrees)
galactic: (264 degrees, +48 degrees)

the speed that the sun and planets are heading in that direction
is 1.23 thousandths of c
You can get that in meters per second if you want to by multiplying
0.00123 by 299792458 meters per second.
I think it comes to around 370 kilometers a second
so it is more than ten times faster than the earth goes around the sun
but the main thing is to know it as a fraction of the speed of light
because that tells you right off the rough size of the doppler effect
on the microwaves, which is the measurable thing.

[YT2095]

this one`s a fave of mine.

http://www.fourmilab...p&img=irsat.bmp

taken from homepage: http://www.fourmilab...ew/vplanet.html

Enjoy :))

[Martin]

YT and others who have chipped in, thanks!
it's great to have a collection of astronomy favorites from a bunch of different people, hope more will contribute.

here's some pedagogical links for cosmology:

This article by Lineweaver (he was one of the team in charge of COBE
an earlier CMB satellite observatory)

"Inflation and the Cosmic Microwave Background"
http://nedwww.ipac.c...r_contents.html

http://arxiv.org/astro-ph/0305179

the second link has a PDF version that is more readable but takes more time to download

Lineweaver's essay has been made into a chapter of a book now in press called "The New Cosmology" (world scientific 2004)

I will repost this link to Ned Wright, because of his FAQ which is
famous on the web and has been translated into several languages
IMO it is the overall best online cosmology FAQ

http://www.astro.ucl...ht/cosmolog.htm

http://www.astro.ucl...mology_faq.html

he teaches the undergrad and graduate level courses in cosmology at UCLA
and is also one of the team in charge of the WMAP satellite observing the CMB

[ydoaPs]

http://www.extrasolar.net catalog of extrasolar planets including minimum mass, distance, and system

[Martin]

Earlier I posted the Friedmann equations, this version includes the cosmological constant as "dark energy" in the rho term. this is how a lot of people do it nowadays, and the dark energy fraction is given as 73 percent of total energy density rho.





Sometimes it's good to be able to separate the cosmological constant part out as Lamda, an inverse distance squared term. then rho is all the other stuff, not counting dark energy, and the equations are:





BTW all this is according to Sean Carroll Living Review article
http://relativity.li...01-1/node3.html

this is with c = 1 units, which simplifies things some.
the scale factor of the metric (whose increase is the expansion of the universe) is denoted by the letter a.
k is a spatial curvature parameter used to distinguish three cases
k = -1, 0, +1 for negative curvature, spatially flat, positive curvature

rho is an energy density, and easy to confuse with p pressure

the universe appears to be spatially flat, the critical density rho_crit is that needed for it to be perfectly flat with k = 0

the Hubble parameter H is defined to be the time derivative a' of the scale parameter a, divided by a.

for the time being assume we've included the Lambda term in rho as "dark energy, because this is a convenient way to set things up for calculating stuff, like the critical density. In the case of a spatially flat universe the first Friedmann equation boils down to



algebraically that turns into the formula for the critical density



the Hubble parameter has been measured really accurately at 71 km/s per Mpc
and this lets us calculate the critical density at 0.83 joule per cubic km.since the U tests out flat or very nearly so, this is taken to be the
density of all the stuff, stars galaxies, light, dark matter, dust, dark energy etc. It all amounts to 0.83 joule per cubic km.

And the dark energy being 73 percent (from supernova data) means that its share is 0.6 joule per cubic km.

Sean Carroll is a blogger as well as one of the worlds foremost cosmologists. he's at chicago
check out his blog sometime--it can be entertaining
the name is "preposterousuniverse"

the albert einstein institute near Berlin has charge of LivingReviews of Relativity and they asked Sean Carroll to do their article on Cosmological Constant---I chose to follow Carroll's notation because it's standard.
Dont always like what Carroll says or agree with him but its an authoritative source,which reduces chances of confusion.

[DarthDooku]

This may be obvious to subscribers of the Astronomy magazine and newsletter, but they have totally redone the site. It is much more professional, and they have added a lot more features. There is one thing where they have a live chat and showing of some celestial objects. If you subscribe and dont know about it, check out the new site. Or buy a copy at the newsstand and you can gain access to all the subscriber features.

Link:

www.astronomy.com