the sky

modern science was born when people tried to understand the sky

(With thanks to Mark Ritter & John Bloom)

The stars

• first let’s organizethe sky

Constellations

In ancient times, constellations only referred to the brightest stars that appeared to form

groups, representing mythological figures.

constellations

• the ones we know started in ancient Mesopotamia, then went to Babylon, Greece, Rome…

Constellations

Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

• there are 88 of them• there are also things called

asterisms; groups of stars that usually look like something

• like the Big Dipper, and the Great Square of Pegasus

• most constellations and asterisms are made of stars that are not physically associated with each other…

The stars of a constellation only appear to be close to one another

Usually, this is only a projection effect.

The stars of a constellation may be located at very different distances from us.

the names of stars

• most constellations

are in Latin, but most stars derive their names from…

• Arabic• e.g. Betelgeuse came

from yad al-jawza, the giant’s armpit

• names don’t say much and we run out of them soon

• another way is to name them with greek letters from a on (alpha usually is brightest, beta next, and so on)

• so it is Greek letter + NAME• e.g.:

Centauri,b Gemini,g Canis Majoris

The Magnitude Scale

First introduced by Hipparchus (160 - 127 B.C.):

• Brightest stars: ~1st magnitude

• Faintest stars (unaided eye): 6th magnitude

More quantitative:

• 1st mag. stars appear 100 times brighter than 6 th mag. stars

• 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

The Magnitude Scale

Sirius (brightest star in the sky): mv = -1.42Full moon: mv = -12.5

Sun: mv = -26.5

The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects):

• moreover, all these are just how they look at visible wavelengths

• (apparent visual magnitude mv is another

name for it)• but what about those that

pour out uv or x-rays?• and what about eyes that

are more sensitivethan yours?

the skyand its motion

• we have to go back into the past to get a feel for the sky now…

the celestial sphere

• ancient astronomers believed the sky was a great dome, with stars stuck on it

• of course, it’s not, but it’s still convenient to see it that way

• why did people make this model of the cosmos?

• because it looks like it!

• the whole sky like a giant sphere seems to travel around us

• ready for some vocabulary?

• zenith & horizon

• angular distance is measured in

degrees• 1/60 of a degree

is an arc minute

• 1/60 of an arc

minute is an arc second

• here are simple ways to measure

• circumpolar constellations are ones that never go below the horizon

• at NP, all are cp,• at equator , there

are none…• we have a few

• what we see depends on where we are

• see that the angle the NCP is above horizon is ourlatitude

precession

• Hipparchus first noticed that the NCP wasn’t fixed; it was slowly moving!

• Earth spins around like a top

• Its wobble is called

precession• takes 26,000 years!

• this is how the sky was when Thuban was North Star (~3000 BC)

• the Sun & Moon pull on our wider equatorial region so we don’t wobble over

• it’s the perfect wobble!