review for second midterm - texas a&m...

Review for second midterm

Fermi’s paradox

If there are so many planets out there, at least some of them should have advanced civilizations that could have colonized much of the galaxy. So what haven’t they contacted us? There are a few possibilities:

1. We are alone.2. There is a galactic civilization, but they haven’t

contacted us yet.3. Civilizations are common, but none have colonized

the Galaxy.

Fermi’s paradox

• If civilizations are common and if none have colonized the Galaxy, what are the implications for us?• Interstellar travel is just too difficult.• The desire to explore or to make contact with other

civilizations is not that common.• Intelligent life is short-lived; we will destroy

ourselves, or be destroyed by some natural catastrophe.

Topics we’ve covered since the last midterm

• The nature of light

• Stars- stellar classification- stellar evolution- the end stages of stars

• Extrasolar planets

• Life in the Universe

Light: diffraction

Diffraction reveals that light can be made up of some combination of many different colors, or only one color. This is shown by looking at it’s spectrum.

Blackbody Radiation 57

Part I: Spectral Curves V

White light is made up of all colors of light. We can see the individual colorswhenwhite lightis passed through a prism or when we look at a rainbow. Light’can come,in: an array of typesor forms, which we call a spectrum. A spectral curve (like the one:shown:.below) isa graphthat displays the amount of energy given off by an object each second’versuSthe differentwavelengths (or colors) of light. For a specific color of light on the horizontal axis, the heightof the curve will indicate how much energy is being given off at’that particular wavelength..Figure 1 shows the spectral curve for an object emitting more red and orange !ight thanindigo and violet. Notice that the red end of the curve is higher than the, yioletend so theobject will appear slightly reddish in color.

1) Which color of light has the greatestenergy output in Figure 1?

2) Imagine that the blue light and___—

orange light from the source wereblocked. What color(s) would now bepresent in the spectrum of light observed?

I I I I I I IV I B G Y 0 R

3) Which of the following is the most Violet Indigo Blue Green Yellow Orange Redaccurate spectral curve for thespectrum described in Question 2? Figure 1

a) b) c)

0 0 0C) C) C)

VIBGYOR VIBGYOR VIBGYOR

4) What colors of light are present in 3b above?

5 What colors are resent in 3c above? Would this obect appear reddish or bluish?

©r2QO8iearson Education, Inc., LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY

PubIishingas Pearson Addison-Wesley. SECOND EDITION









a) b) c)

0 0 0C) C) C)














a) b) c)

0 0 0C) C) C)






Light: light is a waveLight behaves like a wave, where it’s color is determined by the wavelength (or the frequency)

wavelength — the distance between adjacent peaks

frequency — the number of times each second that a point moves up and down

When wavelength goes up, frequency goes down:frequency = c/wavelength

Light: the different wavelength regimes

The human eye is only sensitive to a narrow range in wavelength or frequency. That’s why you can’t see light with shorter wavelengths (like x-rays) or longer wavelengths (like infrared and radio).

Light: the thermal spectrumDense objects emit light with a thermal spectrum that depends on their temperature. Hotter objects are brighter at all wavelengths, and peak and a shorter wavelength

Light: it also behaves as a particle

Light is emitted or absorbed in discrete packets. In other words, even though we know light behaves like a wave, it also behaves like a particle. We call these particles photons.

energy of a photon: E∝f

AtomsAtoms consist of a nucleus of protons and neutrons surrounded by a cloud of electrons. The type of element is determined the number of protons, also called the atomic number. Sometimes atoms of a particular element can have different numbers of neutrons; these are called isotopes.

Atoms: electron energy levelsElections can occupy different energy levels. If an electron gains or loses the exact right amount of energy, it can transition between the levels. And if it gets enough energy then it can escape the atom entirely.

(the ground state)

n=5

n=2

n=4

n=3

n=1

Light: the three different types of spectra

Emission line spectrum:

Continuous spectrum:

Absorption line spectrum:


A dense object will emit a continuous spectrum. This will depend on it’s temperature, which is why it’s also called a thermal spectrum (also a blackbody spectrum)


A hot cloud of gas will emit an emission line spectrum. Because of the thermal energy in the gas, some of the electrons will get bumped up to higher energy levels. When they move back down to lower energy levels, they will emit photons corresponding to the difference in energy.


A hot dense object illuminating a cool cloud of gas will produce an absorption line spectrum. Most of the electrons in the gas will be in the ground state. But photons from the source that have exactly the right amount of energy will be absorbed by the electrons, moving them into a higher energy state.

Light: figuring out what element you’re looking atDifferent elements have different electron energy levels. So by looking at the wavelengths of the emission/absorption lines, you can figure out what element you’re looking at!

Light: Doppler shift

The Doppler shift refers to the change in wavelength and frequency of a wave if the emitting object is moving towards or away from you.

The waves get bunched up if the object is moving towards you, or stretched out if it is moving away

Light: Doppler shift

Stationary

Moving away

Away faster

Moving toward

Toward faster

Redshift

Blueshift

Light: Doppler shift• By measuring the shift in the spectrum, we can estimate

very accurately the speed with which an object is moving towards or away from us.

• But this only gives us the speed along the line-of-sight

Large redshift

No shift at all, since it is moving perpendicular to the line-of-sight

Small redshift, since it is moving away from us but not very quickly

Stars: brightness and luminosity

• The apparent brightness (or just the brightness) of an object is how bright it appears to us, or how much radiative energy we receive.

• The luminosity of an object refers to how intrinsically bright an object is, or how much total radiative energy it gives off.

apparent brightness = luminosity4𝛑×distance2

Stars: spectral typesStars have absorption line spectra — there is a bright thermal continuum with absorption lines due to the cooler stellar atmospheres

O

B

A

F

G

K

M

Hotter stars have bluer spectra, and have no absorption features

Cooler stars are redder and have lots of absorption features

Stars: the Hertzsprung-Russell diagram

If you plot stars on a graph of luminosity versus temperature, then you see some very clear patterns emerge


Stars on the upper left of the main sequence are hotter, (and consequently bluer) and more luminous. Stars on the lower right are cooler, redder, and fainter.

Giants and supergiants are very luminous, but have relatively low temperatures. So we know that they must be very large.


Review

A star’s spectral type simply depends on its surface temperature

Stars: formation

Recall the “nebular theory” for how the Sun formed. Well, we expect that the process is basically the same for all stars.

• Stars form out of large gas clouds (“nebulae”)• As a cloud undergoes gravitational collapse, it heats

up and flattens into a spinning protoplanetary disk.• At the center is the protostellar core which becomes

very hot and dense

Stars: formation

When nuclear fusion begins to take place, we call it a star.

For as long as hydrogen is present in the stellar core, it fuses into helium. Because the resulting helium atom contains slightly less mass than the original four hydrogen atoms, energy is released according to E=mc2

The H-R diagram

Stars spend most of their life on the main sequence of the Hertzsprung-Russell diagram, for as long as they are fusing hydrogen into helium in their cores.

The fusion rate depends strongly on the mass of the star; the most massive stars (>100 Msun) burn their fuel very rapidly, so they are very hot and don’t live long.

The H-R diagram

Review

Low-mass stars (down to ~0.1 Msun) burn their fuel very slowly, so they are cool and live very long.

After the core of the star has been converted into pure helium the fusion stops (although it will still occur in a shell around the core). The star leaves the main sequence, expanding to become a red giant or a supergiant.

The H-R diagram

Stars: the giant phase

Once all the hydrogen in the core of a star has turned into helium, then hydrogen can only burn in a shell around the helium core. The star starts expanding, ultimately to ~100 times it’s normal size, becoming a red giant.

Stars: the giant phase

Giants, and supergiants:

Stars: how they die

So what happens when all fusion finally stops in a giant star? If M<8Msun, then:

• The core (which is now made of either helium or carbon) contracts and heats.

• Through strong stellar winds and other processes, the diffuse out layers of the star are ejected creating a planetary nebula.

• The dense stellar core remains, and is now called a white dwarf.

How stars die — when M<8Msun

Hot ionized gasejected from the star

white dwarf

Stars: how they die

So what happens when all fusion finally stops in a red giant? If M>8Msun, then:

• The core (which is now made of heavier elements, but not heavier than iron) undergoes a sudden gravitational collapse.

• The outer layers of the star explode outward in a supernova.

• The dense stellar core remains, and is now called a neutron star. Or, if the core is massive enough, it’ll become a black hole.

How stars die — when M>8Msun

Hot ionized gasejected from by the supernova

neutron staror black hole

The crab nebula: the supernova was observed in 1054

Stars: stellar remnants

• White dwarfs are supported by electron degeneracy pressure

• Neutron stars are supported by neutron degeneracy pressure

• Black holes seem to be infinitely dense. But we can’t actually tell, because it’s hidden by the event horizon

Chemical evolution of the universe

We are made of starstuff — Carl Sagan

They can be found by direct detection, or by indirect detection: There are three methods of indirect detection:

• The astrometric method — the position of a star shifts periodically

• The Doppler method — we see a periodic Doppler shift

• Transit method — the planet passes in front of the star

Extrasolar planets

Extrasolar planets

The transit method:

The astrometric method and the Doppler method involve detecting gravitational effect of the planet on the star (i.e. since both the planet and the star orbit their mutual center of mass, we can see the star moving):

Extrasolar planets

Extrasolar planets

Doppler method: we observe the Doppler shift of the star as it moves in it’s orbit

Extrasolar planets

These methods all have drawbacks:• The transit and Doppler methods only work if the

orbit is seen edge-on, or close to it.• The astrometric and Doppler method work best if the

planet is very massive and close in.• The transit method works best if the planet is very

large.

So many of the planets that we’ve detected so far can be classified as hot Jupiters… and these are problematic given our theory of planet formation, since we expect massive gaseous planets to form outside the frost line.

Life in the Universe

Evidence suggests that life on Earth occurred more than 3.5 billion years ago, shortly after the period of heavy bombardment

This suggests that life does not arise due to some very rare and random process… therefore life may be pretty common! But intelligent civilizations took much longer to develop, so they may be much more rare in the Universe.


All of the plants and animals are just a very small part of the tree of life here on Earth. We should realize that other forms of life are probably much more common throughout the Universe


Not all life is as delicate as we are. Extremophiles can live in very harsh conditions.

Life in the UniverseThe search for civilized life on other planets begins with the search for planets in the habitable zone — they must be the right distance from the star to have liquid water. The range of allowable distances depends on the type of the star:


One of the very important characteristics of Earth for life is it’s stable climate, which is due to the greenhouse effect.

Life in the Universe — can we find it?SETI — Search for Extraterrestrial Intelligence — uses radio telescopes to search for signals from space.

But any signals emitted from other planets would have to be very powerful to be detected using our current technology. But the technology is improving…

Even the nearest stars are lightyears away, so if we’re ever going to get there in a reasonable amount of time we need to come up with some completely new technology.

Life in the Universe — can we travel to it?

review for second midterm - texas a&m...

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