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# Distances - PowerPoint PPT Presentation

Distances Near objects appear to move more than far objects against a distant horizon. Trigonometric parallax is used to measure distance to near stars. Parallax time A r y Earth’s orbit d distant stars time B near stars

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### Distances

Near objects appear to move more than far objects against a distant horizon.

Trigonometric parallax is used to measure distance to near stars.

Parallax

time A

r

y

Earth’s orbit

d

distant stars

time B

near stars

Define distance by angle

The parsec (pc) is the distance that would result in one arc second of parallax.

1 pc = 3.086  1016 m

1 pc = 2.06  105 AU

Parsec

Near Stars angle.

• The Hipparcos satellite measured parallax of 118,000 stars.

• Resolution: 0.001 arc-second and 0.2% luminosity.

• Gaia launches in 2011 to measure 109 stars in the galaxy

Arcturus 11.3 pc (ESA)

Use for absolute magnitudes

Precise HR diagram

Distant stars can be fit on the main sequence.

Measure luminosity and apparent magnitude

Spectroscopic Parallax

Better to average stars at the same distance

Globular clusters are dense with 100,000 stars in a 20-100 pc region with less than 0.3 pc separating the stars.

Open clusters tend to be smaller and younger.

Star Clusters

Type I Clusters angle.

Hot young stars

Lots of gas and dust

Abundant in heavy elements

Active star formation

Type II Clusters

Old red stars

No gas and dust

Few heavy elements

No star formation, just old stars

Cluster Types

Standard Candle angle.

• Up to 30 pc distance is measured with parallax.

• Less certainty to 300 pc

• Longer distances by spectroscopic parallax

• The best measure of large distances are variable stars.

• Luminosity directly related to the period.

Radial pressure waves move at the speed of sound.

The period of vibration is inversely proportional to the square root of the density.

Vibrational Modes

~ 4 to 15 M

Color classification F to K

The period and apparent luminosity determine the distance.

Density ~ 10-3 kg/m3

Vibrational period ~ 106 s

Cepheid Variables

~ 1 M

Color classification A

These are type I stars.

Found in globular clusters

Useful for galactic distances

Density ~ 10 kg/m3

Vibrational period ~ 4  104 s

RR Lyrae Variables

Variables in M3 energy.

RR Lyrae stars in one night time lapse

Instability strip

Not on main sequence

As stars pass through band they oscillate.

Instability Strip

Observed as a diffuse band

Millions of stars in a telescope

The Milky Way is thicker in some directions.

Appears as a band across the sky

Band of Stars

Disk diagram.

• The band of the Milky Way is the same view a viewer would have sitting inside a disk of stars.

• This disk has type I stars with gas and dust.

sun

sun

top view

side view

Halo diagram.

• Astronomers measure the distance to globular clusters.

• Type II globular clusters are in a sphere around one point.

• This sphere is the galactic halo.

• The center of the sphere is the center of the galaxy.

sun

globular clusters

Size and Shape diagram.

• To view the galaxy from inside, we

• measure the distance to globular clusters

• measure distributions of hydrogen gas in the disk.

• The Milky Way is 50,000 pc across with a central bulge.

• The stars group in arms.

Galactic Structure diagram.

• The galactic nucleus is bright and massive.

• It is obscured by the dust of the galactic disk.

• The Milky Way is probably similar to M83.