A1199Are We Alone?

The Search for Life in the UniverseSummer 2019

Instructor: Shami ChatterjeeWeb Page: http://www.astro.cornell.edu/

academics/courses/astro1199/Today: Measuring the Universe

Are We Alone? The Search for Life in the Universe

Understand, discuss, speculate about:• The nature of the observable universe.• Terrestrial life in the context of modern astrophysics.• Prospects for life existing elsewhere.• Exploration and remote sensing (solar system, cosmos).• The future of terrestrial life (esp. H. Sapiens).• Methods and status of science.

The Search for Life in the Universe

What the course is about:• Where, when, and how we may find life elsewhere.• Three big words:

Universe: The big picture, fundamentals, profound facts.Life: Is finding bacteria as good as finding counterparts to us?Search:How will we find, detect, or infer life elsewhere?Space probes vs. remote sensing.

THE DISTANCE LADDER

“… Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space…”

The Radius of the Earth

Early estimates: Eratosthenes (276-194 BC).

Compare Sun’s elevation at noon on solsticeat Alexandria and Syene+ basic trigonometry.

Modern value ~6400 km.(He got to within ~16%.)

The “Astronomical Unit”

Earth-Sun distance:Early Greeks, Chinese: Different shadow lengths at noon give distance to Sun, assuming a flat Earth.Aristarchus of Samos, “On the Sizes and Distances of the Sun and Moon”:

• Compare angular sizes of Sun, Moon.

• Compare Moon size vsEarth’s shadow at eclipse.

• Angle at half-moon is nearly 90 degrees.

• Got large distance, but too low by 60x.

The “Astronomical Unit”

Earth-Sun distance:

Richer and Cassini – parallax to Mars between Paris and French Guiana in 1672.

With relative orbit sizes from orbit periods, get AU value.

The “Astronomical Unit”

Transits of Venus• Pairs of transits every 8 years, separated by 121.5 and 105.5 years.

(Most recent in June 2012.)

• Parallax of Venus from observing transit ingress/egress at different sites + relative orbit sizes of Earth and Venus = get AU value.

• First in 1639, then 1761 and 1769 through the Seven Years’ War.

1 AU = 149 597 870 700 m~ 150 million km.

The Speed of Light

Early estimates: Ole Roemer,Danish astronomer (1644-1710),by timing the eclipses of Jupiter’s Galilean moons.

c = 299 792 458 m/s~ 3 x 108 m/s or 300,000 km/s.

(Since 1983, the meter is defined in terms of this speed.)

ParallaxPurely geometric method to determine distance.

Model-independent,hence very valuable,but hard to measure.

ParallaxExample: With the VLBA, we observe a radio pulsar moving across the sky.

μα = 94.09 ± 0.11 mas/yrμδ = 42.99 ± 0.16 mas/yrπ = 2.77 ± 0.07 masèD = 1/π = 361 ± 10 parsecs.

1 parsec = distance at which parallax is 1 arcsec for 1AU.Since 1” = π/(180*60*60) radians = 1/206,265 radians,

è 1 pc = 206,265 AU = 3.08567758 × 1016 m.

Parallax

Hipparchus of Nicaea (190-120 BC; painting by Raphael)

ESA Hipparcos (1989-1993)

Gaia (ESA)A global space astrometry mission, Gaia is making the largest, most precise three-dimensional map of our Galaxy by surveying more than a billion stars.

Inverse-Square Laws

Area of a sphere= 4πR2.

Works for gravity, electric field, magnetic field, flux of light from a light bulb or a star, etc., etc. = Very general principle.

Conservation of flux (of whatever)implies that it must scale as:

F(R) ~1/R2.

Distances from LuminositiesFc1 = L0/(4πD1

2)Fc2 = L0/(4πD2

2)

D1 = 2D2 çèFc1 = 1/22 Fc2 = ¼ Fc2.

Hertzsprung-Russell diagram, illustrating stellar surface temperature, color, and absolute luminosity.

If we can classify a star (based on color, spectral lines) we can (very approximately) infer its distance.

Cepheids and Other Distance Indicators

Cepheid variables: • Very luminous stars that

pulsate in a regular cycle.• Rapid brightening followed

by gradual dimming. • Named after delta Cephei,

a naked eye star, the first of this type to be identified.

• “Eddington valve”.

• Rare, but key to the distance ladder:

The absolute brightness and the pulsation period are related:

M = -2.78 log (P) - 1.35M: absolute magnitude;P: period in days.

Hubble Space Telescope “Key Project” on the extragalactic distance scale: provide a measure of the Hubble constant accurate to 10%.

Sample light curves for Cepheids in M100 (Freedman et al. 1994) .

Doppler Shifts

Change in apparent wavelength (and pitch) due to relative motion of source and detector. Δλ / λ = v/c

Spectral lines

Interaction between a quantum system and a single photon:Photons absorbed or emitted at very specific wavelengths.

Emission spectrum

Absorption spectrum

White light

Lines can be broadened by many phenomena:e.g., collisions, magnetic field effects, thermal Doppler broadening, etc.

Lines provide a fingerprint to identify emitting or absorbing material.

Spectral lines

Spectral lines

Absorption spectrum

What is happening here?

The Doppler shift of spectral lines (emission or absorption) traces thevelocity of the emitting or absorbing material, according to our Doppler shift equation, Δλ / λ = v/c.

So far…

• The radius of the Earth (6400 km).• The Earth-Sun distance (1 AU = 150 million km). • The speed of light (3 x 108 m/s or 300,000 km/s).• Parallax: model-independent distances.• Inverse-square laws: F(R) ~ 1/R2.• Distances from Cepheids.• Doppler shifts, Δλ / λ = v/c.• Spectral lines.

Redshift and Cosmological Expansion

Edwin Hubble observingat the 100-inch telescope at Mt. Wilson, early 1920s.

In 1923, Hubble identified Cepheid variable stars in the Andromeda galaxy.P=31.45 days è 7000 times brighter than Sun.Implied that Andromeda was 300 kpc away.

Follow-up: Distances to many other galaxies…

(Velocities from Doppler shifts of spectral lines;Distances from Cepheids.)

In spite of many systematic errors, the basic trend is clear:

Galaxies that are further away are moving away from us faster.

“A relation between distance and radial velocity among extra-galactic nebulae”- E. Hubble (1929)