C2_night Sky And Star Charts

Knowing the Heavens Chapter 1 sections 5, 6, 7 Chapter 2 sections 2, 3, 4; Box 2-1

Announcements

• Path to Q-drive

10 kpc

Solar System

Galaxy Illustration

Galactic Neighbourhood Solar system

Nearly all ‘stars’ we see at night with the naked eye are located in our own galaxy

Orion Nebula

What we see depends upon time of night and time of year

•The Earth spins on its axis once every 23hours 56minutes •The Earth orbits the sun once every 365.25 days

Eighty-eight constellations cover the entire sky • Ancient peoples looked at the stars and imagined groupings made pictures in the sky • We still refer to many of these groupings • Astronomers call them constellations (from the Latin for “group of stars”)

Modern Constellations • On modern star charts, the entire sky is divided into 88 regions • Each is a constellation • Most stars in a constellation are nowhere near one another • They only appear to be close together because they are in nearly the same direction as seen from Earth

Simple Star Chart

Star charts and maps plot the location of celestial objects.

Landmarks on typical star charts Ecliptic Equator

Summer Solstice

Au t

um na

l a rn

i u q

x o n

E

Ve

lE

qu

in o

Winter Solstice

x

Declination

Date Scale

Right Ascension

Mil

ky Wa y

Equatorial Region Star Chart

Equatorial Region Star Chart

Stars visible in early evening in February

Galaxies, nebula and star clusters

North Polar Star Chart

Equatorial Region Star Chart

Photograph of the Orion region

SkyNews magazine, Jan/Feb 2004 page 11. Photo by Alan Dyer

Orion

+10 degrees

0 degrees = Celestial equator Declination (Dec.)

6 hours

5 hours -10 degrees

Right Ascension (R.A.)

Orion

North

East

Exploring the night sky with software • • • • •

Celestial sphere Horizon and Zenith Altitude and azimuth Stars, nebulae, star clusters, galaxies Right ascension and Declination

Geometry Tools Chapter 1

Angular Measure

• The basic unit of angular measure is the degree (°). • Angular measure describe the apparent size of and distance between celestial objects. • The Moon subtends an angle of ½°.

Angular separation

If you draw lines from your eye to each of two stars, the angle between these lines is the angular separation or distance between these two stars

Simple Celestial Protractor

Simple Instrument

150 200

The adult human hand held at arm’s length provides a means of estimating angles

Angular Size vs. Linear Size

The angular size does not help us know the true size unless the distance is also known.

Other Angular Units • • • •

• Hours • Minutes • Seconds

radians (r), degrees (o), minutes of arc (’), seconds of arc (”).

1 = 180 /π = 57.30 1° = 60 arcmin = 60´ 1´ = 60 arcsec = 60” r

o

o

24 hours = 360 degrees 1 hour = ____ degrees 1 minute = ____ arc minutes

Geometry and Angles c

β

a

a2+b2=c2 α

90

o

b

sin α = a/c cos α = b/c tan α = a/b α + β + 90 = 180

Circles and Angles Circumference C = 2π r s α

r

Arc length, s s=αr r = radius s = arclength α = angle in radians C = circumference

Small Angle Formula

D

If α is expressed in arc radians:

D  d

If α is expressed in arc seconds:

αd D= 206,265

d If α is expressed in degrees α

D  d tan( )

Which is simpler to use and remember?

Distances in Astronomy • Kilometres (km) • Astronomical Unit (AU) – One AU is the average distance between Earth and the Sun – 1.496 X 108 km

• Light Year (ly) – One ly is the distance light can travel in one year – 9.46 X 1012 km – 63,240 AU

• Parsecs (pc) – The distance at which 1 AU subtends an angle of one arc second – 3.26 LY – 206,265 AU

Small Angle Formula Example • The Andromeda Galaxy is about the same size as our own galaxy, 150,000 LY. The angular size of the Andromeda Galaxy is about 3.0 degree. • Determine the distance to the galaxy in light years.

?

αd D= 206,265

D  d tan( )

D  d

Answer: d = 2.9 x106 LY

Brightness of Stars

fainter

Apparent Magnitudes

-27 -12 -5 -1.5 0 0.5 0.9 2 6 29

Sun Moon Venus Sirius Vega Betelgeuse Aldebaran Polaris Naked eye limit Hubble Space Telescope limit

Magnitude System • Absolute magnitude (M) is the magnitude a star would have if the star was located at a standard distance of 10 parsecs. • Apparent magnitude (m) is the magnitude observed at the stars actual distance. • Luminosity (L) is the total amount of power emitted by a star in Watts. • Brightness (b) is the power that reaches the observer diminished by distance, absorption and reflection (Watts per square metre)

Relationship between magnitude and brightness m1 - m2 =2.5 log (b2 /b1) Brightness of Sun (solar constant) = 1370 W/m2 Magnitude of Sun = -27 Magnitude of Moon = -12 What is the brightness of the Moon?

• Magnitude represents the human’s eye’s response to the light from stars • Brightness is the power measured by electronic light sensors. • The faintest star that a human eye can detect is 6th magnitude. What is the brightness of a 6th magnitude star? (assignment question) – This is the sensitivity limit of the human eye – Corresponds to the limit of hearing of the human ear, which is zero dB.

Magnitudes on Star Charts

Legend

Terminology • • • • • •

Star Chart Constellation Celestial sphere Right Ascension (R.A.) Declination (Dec.) Magnitude – apparent – absolute

• Luminosity • Brightness

• • • • • • •

Arc seconds Arc minutes Radians Astronomical units Light Years Parsecs Small angle formula

End of Presentation