ASTR 1102-0022008 Fall Semester

Joel E. Tohline, Alumni ProfessorOffice: 247 Nicholson Hall

[Slides from Lecture19]

Chapter 23: Our Galaxyand

Chapter 24: Galaxies

Schematic Illustration of

Our (Milky Way) Galaxy

Real ‘All Sky’ Imagesof

Our (Milky Way) Galaxy

Aside:

• Atomic transition that gives rise to 21-cm radiation, which is used by astronomers to map out the distribution of neutral hydrogen throughout our Galaxy (and other galaxies), is also the physical principle underlying the MRI (magnetic resonance imaging) diagnostic tool in modern medicine.

Medical MRI

Determining Size of MW Galaxy

• We have not always known that the diameter of our Galaxy is ~ 50 kpc (as illustrated in following slide)

• Herschel’s map of our Galaxy (1785) based on star counts– Thin disk not much more than 1 kpc across– Sun approximately at center of disk

Determining Size of MW Galaxy

• We have not always known that the diameter of our Galaxy is ~ 50 kpc (as illustrated in following slide)

• Herschel’s map of our Galaxy (1785) based on star counts– Thin disk not much more than 1 kpc across– Sun approximately at center of disk

Determining Size of MW Galaxy

• We have not always known that the diameter of our Galaxy is ~ 50 kpc (as illustrated in following slide)

• For example, Herschel’s map of our Galaxy (1785) based on star counts …– Thin disk not much more than 1 kpc across– Sun approximately at center of disk

Herschel’s Map of MW Galaxy

Determining Size of MW Galaxy

• We have not always known that the diameter of our Galaxy is ~ 50 kpc (as illustrated in following slide)

• For example, Herschel’s map of our Galaxy (1785) based on star counts …– Thin disk not much more than 1 kpc across– Sun approximately at center of disk

• Herschel’s map grossly distorted by interstellar extinction

Prominent and Obscured Objects

Shapley’s View of MW Galaxy

• Look out of the plane of the MW disk to minimize obscuration due to interstellar extinction

• Distribution of Globular Clusters not symmetric about Sun’s location

• Distances to GCs obtained using RR Lyrae variable stars as “standard candles”

Shapley’s View of MW Galaxy

• Look out of the plane of the MW disk to minimize obscuration due to interstellar extinction

• Distribution of Globular Clusters not symmetric about Sun’s location

• Distances to GCs obtained using RR Lyrae variable stars as “standard candles”

Shapley’s View of MW Galaxy

• Look out of the plane of the MW disk to minimize obscuration due to interstellar extinction

• Distribution of Globular Clusters not symmetric about Sun’s location

• Distances to GCs obtained using RR Lyrae variable stars as “standard candles”

Shapley’s View of MW Galaxy

• Look out of the plane of the MW disk to minimize obscuration due to interstellar extinction

• Distribution of Globular Clusters not symmetric about Sun’s location

• Distances to GCs obtained using RR Lyrae variable stars as “standard candles”

Determining Distances in Astronomy

• Stellar parallax• Spectroscopic parallax (main-sequence

fitting): – Remember distance modulus:

(m – M) = 5 log(d) – 5– If you know “M” for a certain type of star, then

a measurement of “m” gives you “d”

• Standard candles: Identifiable stars for which you know “M”

Determining Distances in Astronomy

• Stellar parallax• Spectroscopic parallax (main-sequence

fitting): – Remember distance modulus:

(m – M) = 5 log(d) – 5– If you know “M” for a certain type of star, then

a measurement of “m” gives you “d”

• Standard candles: Identifiable stars for which you know “M”

Determining Distances in Astronomy

• Stellar parallax• Spectroscopic parallax (main-sequence

fitting): – Remember distance modulus:

(m – M) = 5 log(d) – 5– If you know “M” for a certain type of star, then

a measurement of “m” gives you “d”

• Standard candles: Identifiable stars for which you know “M”

Determining Distances in Astronomy

• Stellar parallax• Spectroscopic parallax (main-sequence

fitting): – Remember distance modulus:

(m – M) = 5 log(d) – 5– If you know “M” for a certain type of star, then

a measurement of “m” gives you “d”

• Standard candles: Identifiable stars for which you know “M”

Determining Distances in Astronomy

• Stellar parallax• Spectroscopic parallax (main-sequence

fitting): – Remember distance modulus:

(m – M) = 5 log(d) – 5– If you know “M” for a certain type of star, then

a measurement of “m” gives you “d”

• Standard candles: Identifiable stars for which you know “M”

Determining Distances in Astronomy

• Stellar parallax• Spectroscopic parallax (main-sequence

fitting): – Remember distance modulus:

(m – M) = 5 log(d) – 5– If you know “M” for a certain type of star, then

a measurement of “m” gives you “d”

• Standard candles: Identifiable stars for which you know “M”

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

NOTE: Transient Events (in time) also occur

NOTE: Transient Events (in time) also occur

NOTE: Transient Events (in time) also occur

Example Standard Candles

• RR Lyrae variables– Pulsation period of about ½ day– Luminosity is 100 x solar luminosity

• Sun: M = +4.8; let’s call it M = +5 for simplicity• RR Lyrae: M = 0

• “Population I” Cepheid variables– Luminosities range up to 10,000 solar (M = - 5)

– (Pulsation) period-luminosity correlation

• Type Ia supernovae– Luminosity 3 x 109 solar !

Distance Ladder

Shapley’s View of MW Galaxy

• Look out of the plane of the MW disk to minimize obscuration due to interstellar extinction

• Distribution of Globular Clusters not symmetric about Sun’s location

• Distances to GCs obtained using RR Lyrae variable stars as “standard candles”

Stellar Populations

• Pop I

• Pop II

• Pop III

Prominent and Obscured Objects