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How far to a Star ?, Slides of Astrophysics

Texas A&M University (A&M)Astrophysics

Stars & Elementary Astrophysics: Introduction. Press F1 for Help ... How to measure astronomical distances ? ... 4.3 yr nearest star (Proxima Centauri).

Typology: Slides

2022/2023

Uploaded on 03/01/2023

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Stars & Elementary Astrophysics: Introduction Press F1 for Help 1

How far to a Star ?

- Fundamental problem …..

- How far away is a point of light ?

. Is it a 100-watt light bulb ?

. A star as bright as the Sun ?

. A galaxy of stars ?

- Brightness alone is not enough.

- How to measure astronomical distances ?

Discover Slides of Astrophysics Texas A&M University (A&M)

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How far to a Star?

Fundamental problem …..
How far away is a point of light? . Is it a 100-watt light bulb? . A star as bright as the Sun? . A galaxy of stars?
Brightness alone is not enough.
How to measure astronomical distances?

10 11

Astronomical Distances

distance = speed x time
light speed
0.002 s Edinburgh-St.Andrews
0.1 s Earth circumference
1.2 s Earth-Moon distance
8 min Sun-Earth
40 min Jupiter
5 hr Pluto

c = 3 × 10 5 km/s

Stars & Elementary Astrophysics: Introduction Press F1 for Help 4

geometrical method p = parallax angle a = 1 Astronomical Unit (1 AU) = radius of Earth’sorbit around the Sun.

d a

a d p p

a d p

≈

= (smallangle )

p tan

Parallax

The Parsec -new distance unit

3.26 light yr

1 10 m

206265 AU

radian

1 AU

1 arcsec ( 1")

16

= ×

× °

= = ×

p π

d a

Example:

Fast Method: d = 1/ p For p in arcsec = 1 parsec (1 pc) and d in parsec

a d

Limits of Parallax

Ground-based CCDs 0.05” 20 pc
Hubble Telescope 0.01” 100 pc
Hipparcos (all sky) 0.005” 200 pc Today: Nearby Stars Only
2015: GAIA (whole galaxy) 10,000 pc

The Distance Ladder

Different methods
for different supernovae
distances
cepheid variables
stars

• PARALLAX IS THE FOUNDATION

FOR ALL OTHER METHODS

Luminosity

time L =^ energy Units: Joule / sec = Watt (e.g. 100 W light bulb) Solar luminosity:

Flux

Units: Watts / square metre Sun viewed from Earth:

Lsun =^3.^8 × 1026 W

F (^) sun^ =^1380 W/ m^2

time area

energy × F =

Luminosity, Flux

Flux viewed from distance d :

area of sphere = 4 π d^2

F L = 4 π 2