Lecture

The Stars

Distance
● How far is the stars? How far is the celestial bodies? How big is our universe?
● In astronomy, distance is not always stated in meter
● Commonly used distance units:
– Astronomical Units (AU)
– Light Years
– Parsec

● Light years tells the distance of a celestial object
● However, it also tells the time needed to reach the object
● The speed of light is 3 x 108 m/s
● Hence, from Earth to the Moon, light needs 1 sec
● From Earth to the Sun: 8 minutes
● to the nearest visible stars (α-Centauri): 4 years
● The distance to α-Centauri is 4 years
● So, now you see the α-Centauri 4 years ago
● What’s the state of α-Cen right now… ? Wait until 4 years
● When you see the stars, you see the past…

Parallax
● The method of parallax is used in measuring the distance of the nearby stars
● Nearby stars appear to shift back and forth relative to more distant stars as Earth revolves around the Sun (called stellar parallax)
● Stellar parallaxes are very small, measured in second of arc ( 1” = 1/3600º )
● 1 parsec (pc) = the distance to a star whose parallax is 1 second of arc (1”)
● 1 pc = 3.26 light-years

p = parallax
d = Star’s distance to the Sun

Star’s distance (parsec)=

● The measured parallax of α-Centauri is 0.74”, so  -Cen’s distance= 1 / 0.74′ ‘ =1.35 pc=4.4 ly

● The measured parallax of Sirius is 0.38”, what is its distance in light-year?
● Canopus has the parallax of 0.01”, what is its distance?
● Proxima Centauri (the closest star to the Sun) has the parallax of 0.77”, what is its distance?

● Proxima Centauri is the closest star to the Sun, but it cannot be seen since it is very dim
● Sirius is the brightest star in the Sky, but its distance is farther than Proxima’s
● The brightness of a star cannot be used in determining its distance
● The brightness of a star may related to its size

Measuring Star’s Brightness
● Apparent magnitude (m) is a measure of how bright a star appears
● Hipparcus (Ancient Greek Astronomer) classified the stars in 6 classes based on their brightness (1 for the brightest stars and 6 for the dimmest stars that can be seen)
● Modern magnitude scale defines that a firstmagnitude star is exactly 100 times brighter than a sixth-magnitude

Magnitude differences between stars measure the relative brightness of the stars
● It can be calculated from Pogson’s scale: m = magnitude ; I = brightness
● Brightest stars are not always associated with bigger stars, it can be related to their close distance
● It can be calculated from Pogson’s scale:

mA, mB = magnitudo

Ia/Ib = intensity

● Apparent magnitude cannot tell the truebrightness of stars
● If we can line up all stars at the same distance from Earth, we can see how they differ in true brightness
● Absolute magnitude (M) is the apparent magnitude of stars if it were located at 10 pc from Earth
● It can be calculated from the distance modulus (the difference between apparent and absolute magnitude)

m−M =5 log (distance in parsec / 10 )

Star Magnitude
Sun                           -26,80
Sirius                       -1,46
Canopus                  -0,72
Alpha Centauri     -0,27
Arcturus                 -0,04
Vega                          0,03
Capella                     0,04
Rigel                          0,12
Procyon                    0,35
Achernar                 0,46
Betelgeuse                0,70
Altair                         0,77
Aldebaran                0,85
Acrux                        0,87
Antares                     0,92
Spica                          1,00

Semakin besar nilai magnitudo suatu bintang, maka semakin redup tingkat keterangan bintang tersebut.

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