Ask An Astronomer

[plummeting_sloth]

Dear Dr Astronomer Lord

I heard that during the winter solstice (in the northern hemisphere) is when the earth is closest to the sun and the summer one (again, in the north) is when it's furthest away. Does that mean for you southern residing gentlemen that the winters are colder and the summers hotter than us in the north?

[Elomin Sha]

That and the hole in their ozone layer.

[Master Gunner]

Given how far away we are from the sun, and there's only about a 3% difference between the Earth's Aphelion and Perihelion (furthest and closest points to the sun), that has little effect on the temperature and seasons. Checking my copy of Bad Astronomy, the temperature change due to distance works out to about 4 degrees (Celsius). So local weather patterns and ocean currents are likely to have a far greater effect then the distance from the sun. Also the equinoxes do not line up with the Aphelion and Perihelion at all. September 22nd and March 20th for the equinoxes this year, verses around January 3 and July 4 (so about two weeks after the solstices, actually).

[plummeting_sloth]

whoops... sorry, yeah gunner. I meant solstices.

[Elomin Sha]

147million Km
152 million Km

5 million Km could be enough.

[Master Gunner]

http://apod.nasa.gov/apod/ap120312.html

I also take issue with some of their choices of names as well as some of their choices at the largest scales.

Tell us more?

[Fezzul]

Re: Master Gunner

Yes, please

[Lord Chrusher]

http://apod.nasa.gov/apod/ap120312.html

I also take issue with some of their choices of names as well as some of their choices at the largest scales.

Tell us more?

On second thought I only take issue with one of the names. M87 is the central galaxy in the Virgo cluster, the nearest galaxy cluster, and is notable to the massive jet of particles being ejected by the massive black hole at its centre.

.
M87 is the galaxy in the centre of this image.

M87 has several names including NGC 4486, VCC 1316, 3C 274 and Virgo A. The website uses Virgo A, a name that is rarely used to describe it.

With regards to the largest scales I feel that they could have added more. Cen A's massive radio jets (also caused by the galaxy's super massive black hole) which are over a million light years across.


The pink is Cen A's radio emission. The background points are not stars; they are radio galaxies like Cen A but much further away.

They could have included things like the distance to the furthest known galaxy (32 Gly) and the furthest known quasar (29 Gyr). Their Hubble Deep Field is a bit misleading. Only the most distant galaxies observed in the Hubble Deep Field are that far away with most of them being much closer. The also use the light travel time (13 Gly) which is wrong - as the Universe is expanding they should have used the comoving distance which is 28 Gly.

I also doubt some of their sizes They have ω Cen being smaller than M54 when ω Cen has a larger radius.

[Master Gunner]

Thanks!

Next question:
More of my knowledge of Astronomy is from Wikipedia than it should be, but my understanding is that the Virgo Cluster is at the center of the Virgo Supercluster, of which the Local Group is an outlying member. Yet, most "maps" I see of the Supercluster use the Local Group as the center. Is this a typical convention, or just Wikipedia being Wikipedia? Are there diagrams of the Virgo Supercluster centered on the Virgo Cluster/M87?

[UnarmedOracle]

Dear Lord Crusher: Which hydrogen emission line is objectively the best, and why is it H-alpha?

[the_lone_bard]

So I heard that during august 12-13 we will be able to look up and see roughly 100 comets a night.

True? Do we get to see this or just the other side of the world?

[Lord Chrusher]

Dear Lord Crusher: Which hydrogen emission line is objectively the best, and why is it H-alpha?

It depends what you are trying to do and what redshift you are looking at. At low redshift Hα is the easiest way to detect ionised hydrogen. As Hα is the lowest energy hydrogen line in the optical, it is the easiest excited and hence the brightest. As it is the reddest is less effected than the other Balmer lines be interstellar extinction. However if you want to correct for interstellar extinction you would want to observe bluer hydrogen lines such as Hβ and Hγ in addition to Hα. Observing higher order Balmer lines also allows you to learn more about what is driving the ionisation. At 6553 Å, rest frame Hα lies in a relatively easy to observe part of the spectrum - not much sky emission lines or telluric absorption lines while being near the quantum efficiency peak of most CCDs and being a low enough wavelength that fringing is not a problem.

However once you go above a red shift of about z = 0.1 all sorts of
ugly sky lines start getting in the way. By z = 0.4 your quantum efficiency will be rapidly dropping while by z = 0.6 it will be practically zero. At these redshifts Hβ would be a better choice or you could switch detector material from silicon to something like HgCdTe if you want to deal with infrared detectors. The higher order Balmer lines will save you for a while as Hβ drops off the limit of silicon at about z = 1.6.

But help is on the way as the ultraviolet Lyman α redshifts above the atmospheric cut off at z = 1.5. Lyα is an even stronger line than Hα and it can be observed with CCDs out beyond to z = 7.

So I heard that during august 12-13 we will be able to look up and see roughly 100 meteors a night.

True although you will only get the full show from north of Brisbane. Go to as dark a place as far north as you can and after 3 am*, look north.

*This is Australian Eastern Standard Time and this time may be specific to the east coast of Australia.

[Gap Filler]

Curiosity made it to Mars safely this week. Any specific thoughts on Curiosity and its mission?

[Lord Chrusher]

It was pretty awesome. I am glad landed correctly for a few reasons. First it is a huge weight off all the people who worked on it for years. Having a mission fail after you have been working on it for years can be like having a friend dying. Secondly it is good to see the interest this generated. This sort of science relies on public money so have the public engaged increases the likelihood of the funding continuing. Thirdly it is easier to ask for further funding if you can point to past successes.

[Drdiggit42]

My post in the for the lols thread has interested me in your opinion on the universe's shape. Which theory you think is right? Bagel shaped? Soccer ball shaped? Something else?

[Lord Chrusher]

I can give an answer that is both boring and exciting. We are now able study the shape of the universe in detail and the universe is observed to be flat to with in less than one precent. When Stephen Hawking wrote his famous book A Brief History of Time twenty five years ago we had only a rough idea of the size, shape, speed and age of the universe (admittedly the answers to these are all tied together). Today we know these all to with in a couple percent.

[Lord Chrusher]

But apparently from the outside it looks beige.

According to Stephen Fry on QI anyway.

The chair of my review committee was the one who figure that out. As well my supervisor has been mentioned on QI as the authority on definition of a galaxy. In this it was a trick question as astronomers can not agree on what constitutes a galaxy.

[Lord Hosk]

Multipart question.
Assuming that financing is not a factor but practicality of size for fuel and habitation are. How long would it take a starship built with todays technology to reach the nearest planetoid that MIGHT be able to support human life?

[Lord Chrusher]

I assume habital without terraforming?

[Lord Hosk]

Can we tell that level of detail yet?

[Merrymaker_Mortalis]

How can I look up into the blue sky and not shit myself knowing that I am staring into the abyss?
It's been tormenting me my whole life.

Well since Harry Potter and the Goblet of Fire when he's in the maze and his perception is reality is inverted and he sees himself upside down, with the abyss of the sky below.

[Lord Hosk]

How can I look up into the blue sky and not shit myself knowing that I am staring into the abyss?
It's been tormenting me my whole life.

Well since Harry Potter and the Goblet of Fire when he's in the maze and his perception is reality is inverted and he sees himself upside down, with the abyss of the sky below.

http://www.youtube.com/watch?v=EgHh3WaCOpw

[Dutch guy]

How can I look up into the blue sky and not shit myself knowing that I am staring into the abyss?
It's been tormenting me my whole life.

Well since Harry Potter and the Goblet of Fire when he's in the maze and his perception is reality is inverted and he sees himself upside down, with the abyss of the sky below.

XKCD seems to have wondered that too just a few comics ago.

Also, somehow I get very dizzy when looking up at the sky when standing. (It's fine when lying on my back or sitting down, usually)

[Merrymaker_Mortalis]

When I look up I feel like I am going to fall. I don't know where, but I fear that I will fall.

Its either feeling that I'm being sucked upwards or I fear I might fall down something because I am not looking where I am going.

Perhaps my fear derives from if I was young I'd look up at the sky when walking and trip :<

I just fear infinity.

[Master Gunner]

I take comfort in the local distortions of spacetime by large masses. Infinity is not worth thinking about on a casual basis.