Astrophysics from Space

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Lecture 4: The extragalactic distance scale. Prof.. . Dr.. M. Baes (. UGent. ). Prof.. . Dr.. C. . Waelkens. (KUL). Academic year 2015-2016. The expanding Universe. One of the most important cosmological discoveries: . ID: 233399 Download Presentation

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Astrophysics from Space

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Astrophysics from SpaceLecture 4: The extragalactic distance scale


Dr. M. Baes (UGent)Prof. Dr. C. Waelkens (KUL)Academic year 2015-2016


The expanding Universe

One of the most important cosmological discoveries: the Universe is expanding.





Edwin Hubble


The expanding Universe

One of the most important cosmological discoveries: the Universe is expanding.


Hubble’s law

Hubble’s first determination resulted in H


= 500 km/s/MpcResulting age of the Universe: about 2 GyrRadioactive dating of Earth rocks (> 3Gyr)


Hubble’s constant

1960s – 1970s:

two groups advocating two distinct values (50 and 100 km/s/Mpc)


Cosmic distance ladder

Each rung of the ladder provides information that can be used to determine distances at the next higher rung.


Fundamental distance measurements

Methods in which distances are measured directly, with no physical assumptions about the nature of the object.Fundamental distance: scale of the Solar SystemAstronomical Unit (AU)Kepler’s third law: if we knowthe distance to one planet, all distances in the Solar System are known


Planetary parallax measurements

Late 1800s: efforts concentrated on Venus at inferior conjunction (in particular during Venus transit)

Early 1900s: Mars and Eros


Radar echo measurements

From 1960s: radar echo measurements on Venus and other nearby planets and asteroidsAU = 149 597 870 691 ± 30 m


Stellar parallax measurements

High astrometric precision is necessary to measure large distancesHipparcos: precision of milli-arcsecaccurate distances out to several 100 pcalso accurate distances to next distance ladder objects (Cepheids)

Contribution of


to the cosmic distance ladder has been crucial


Standard candles: cepheids


Standard candles: cepheids

1908: Henrietta Leavitt discovers period-luminosity relation in cepheids (studying the LMC and SMC)


: Harlow Shapley uses cepheids to determine the size of the Milky Way


: Edwin Hubble uses


to determine distance to Andromeda


Standard candles: type Ia SN

White dwarfs in a binary system, where


matter pushes it over the Chandrasekhar limit.

Consequence: thermonuclear explosion.


Standard candles: type Ia SN

The width of the light curve correlates with the peak luminosity.Huge advantage: type Ia supernovae are extremely bright.



supernovae are ideal standard candles for cosmological studies


Standard candles: caution…

Calibration issues: what is the absolute magnitude(requires thorough definition of a class and enough members in that class)Confusion with similar objects (different SN types, novae versus supernovae…)Interstellar extinctionHow standard are standard candles ?For example: there are two classes of cepheids….

Walter Baade


Secondary distance indicators

Indirect distance indicators, often based on statistical relations in galaxies. To be used when no primary indicators can be used.

Prime example: Tully-Fisher relation


HST Key Project

HST Key Projectslarge observations projects with significant impactseparate time budget (no competition)guaranteed completion in the early years

HST Key Project on the Extragalactic Distance Scale

measure direct distances to 25-30 galaxies using



use these distances to calibrate secondary distance indicators (TF) to probe structure of the Universe


HST Key Project

Spectacular results obtained as soon as the optics were refurbished (late 1993).

8 observations of each target galaxy to detect


with periods of 10 – 50 days.

E.g. more than 80


discovered in M100.


HST Key Project

Final result based on cepheids and cepheid-calibrated secondary methods: H0 ≈ 72 ± 8 km/s/Mpc

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