merrimack college astronomy 1101

Merrimack College Astronomy 1101

Spring 2017

Ralph P. Pass

Stakehenge Results• Section A • Best stake placement, South, 0 degree error, Ali

and Amanda • Best team measurement, Hannah and Chris, 23

degree error

• Section B • Best stake placement, South, 0 degree error, Dan,

Sam, Julia

• Best team measurement, Robyn and Nora, 37 degree error

America Stonehenge Accuracy• Section A • Two groups • Total errors 129 and 142 • So, Rachel Palmer, Nicole Dupre, and Oliva Boudreau

get bonus points

• Section B • Five groups

• Errors ranged from 75 to 220 (220 was a real blunder, if done like the others would have been 71 to 156)

• So, Dan Resop, Connor O’Brien, Josue Ruiz-Maya, and Julia Heffernan get bonus points

Some Really Weird Stuff!

Quasars• Discovered by Radio Astronomers in the

early 1960s

• Before interferometry so poor location estimates

• Eventually we could get good locations and use visual observations.

• Brightest is 3C273, it is the most distant object you can see with a reasonable sized telescope

Quasars – II• Very ‘bright’ at radio wavelengths

• Very compact visually (almost stellar)

• Named Quasi-stellar objects or quasars

• As luck would have it, one of the first found was near the ecliptic

• It was occulted by the moon

• This allowed precise location determination

High Precision Location

Moon before

Moon after

Quasar

Quasars - III• Optically very small

• Large redshift

• Debate over whether they were galaxies or something else (whether Hubble’s law applied)

• If galaxies, Hubble’s distance/redshift relationship, and the observed redshift, then they were very far and hence intrinsically very bright

• Brighter than any other known galaxy9

Quasars - IV• Quasars also varied their output on timescales

of the order of days

• This meant the energy source was very small (size limited by the distance light could travel in the period of variability)

• We now believe that these are massive blackholes, sucking in matter and converting it to energy

• This process is much more efficient than nuclear fusion!

Quasars - V

Paradoxes• Non-scientists think that paradoxes show

that science it flawed

• However, the opposite is true: it shows that science is working

• Scientists are thrilled to find a paradox and even more thrilled to solve a paradox!

Paradoxes• Speed of Light

• Age of Rocks

• Age of Sun

• Age of globular clusters

• Rotation rates of galaxies

Speed of Light• Classical physicists thought light behaved

like other, observed behavior. Namely, if you are traveling at half the speed of light, then shining a light in the direction of travel would result in the light traveling at 1.5 times the speed of light

Michelson and Morley• Showed speed of light was constant

independent of the speed of the emitter or the receiver

• Used by Einstein to lay the ground work for his theories of relativity

Rocks• Scientists discover radioactivity

• Each element/isotope has a specific decay rate

• measured as half-life, the time to convert half of the material to something else

Rocks• Rocks on Earth date to about 4B years ago

• How is this consistent with a cosmology that says the age is 6,000 years?

The Sun

Sun’s Power Source• Ancients thought it was wood

• 19th century scientists thought it was coal

• And concluded it would burn for 20,000,000 years (assuming you knew where to get the oxygen)

• How is this consistent with the Earth being 4.5 billion years old????

Sun’s Power Source• High Pressure means High Temperature means High Speeds for

atoms • Initially this was the result of gravitation

• High speed and pressure allow for atoms to ‘fuse’ • This is the basis for a Hydrogen Bomb • Four hydrogen atoms fuse in a three step process to form

helium (the next element up) • There is a small decrease in mass when this happens • And Einstein found that • E = mc

• Small mass yields large energy (e.g., a paperclip converted to energy is 20,000 tons of TNT, equivalent to the Hiroshima bomb)

Sun Fun Facts• Surface is 5,500K

• Center is 15,000,000K

• So dense, it takes radiation 400,000 years to get from the center to the surface (400,000 miles) and then 500 seconds to get from the surface to the Earth (93,000,000 miles)

Globular Clusters• Using the HR diagram we could tell some

clusters were 12 – 15 billion years old

• Cosmology models up to about 1990 said the age of the universe was 7 – 10 billion years

• How is this possible????

Rotation Rate/Speed• Consider a coin, rotate it, pick an arbitrary

distance and compute the speed of a point that far from the center

Rotation Rate/Speed• Consider a coin, rotate it, pick an arbitrary

distance and compute the speed of a point that far from the center

• 2πr/p

• Speed increases as the radius does

Rotation Rate/Speed• Planets in orbit about a star

• Given a distance, what is the speed?

Rotation Rate/Speed• Planets in orbit about a star

• Given a distance, what is the speed?

• Use Kepler's third law

• p2 = a3

• 2πr/p = 2πa/a3/2 = 2πa-1/2

• so, speed decreases as you increase distance

Galaxy Rotation• Expect star speed around the center to

decrease like the speed of planets decrease

• Do not expect the speed to increase or stay the same

The Paradox• Using multi-spectral observations we can

estimate the amount of radiating matter we can observe

• Using orbits (gravity) we can estimate the total amount of mass (source of gravity)

• The two are significantly different

Science• Puzzles like this are NOT a criticism of

Science or the Scientific method:

• They are how science works and an opportunity to actually do NEW science.

Rotation Curves• Orbital speed at a given radius is a direct

measure of mass inside the orbit

• Symmetric mass outside the orbit does not count!

• So astronomers try to measure the speed at various radii

• Vera Rubin was the point person on this

Observation versus Prediction• Note the

observed velocity is about 5 times the expected velocity

Actual data

Showing the Knowns and Unknowns

Dark Matter to Observable Matter• Look at the observed/predicted velocity chart

again, the ratio of observed to predicted speed is 5

• If only we could relate this to mass…

• It can be done, but we have left this math out of the course!

• GM = rv2

• So if v is constant, the amount of mass inside r is linearly related to r

More interestingly• If velocity is five times faster than expected

for a given radius, the amount of mass is 25 times higher!

But is that all the evidence for dark matter?• Fritz Zwicky was studying galaxy clusters in

• Measure the mass of the cluster from the visible radiation, estimate the velocities, measure the velocities, and compare

• Mass must be 100’s of times larger than what we see!

• It was Fritz that dubbed this “dark matter”

More parts of the spectrum• X-Ray, UV, IR, Radio observations increase

the amount of observed matter

• But STILL a shortfall by a factor in the 10’s

Returning to our Friend Albert Einstein• Remember the deflection of light by the Sun during

a total solar eclipse…

• Mass causes the deflection of light (radiation), so….

• He predicted not only the Sun but galaxies would deflect light as well

• Example of a gravitation lens (as compared to glass lenses)

What Might Happen

Einstein Cross

Einstein Arcs or Rings

Estimate the Mass to Cause the bend• Confirms existence of dark matter and the

amount is consistent with Zwicky’s observations

What is the dark matter?• Several possibilities

• Hard to detect objects • Planet sized objects, ‘dead’ white dwarfs,

solitary neutron stars, solitary black holes

• So little radiation they have escaped detection

• Named Massive Compact Halo Objects (MACHOs)

• Could be detected if passing in front of a star

• Light dimming or gravitation lens effects

Leading (?) theory• Hard to detect particles

• Neutrinos are very very hard to detect and have a very small mass (compared to other atomic particles)

• Not enough to account for all the observed dark matter

• Hypothesized weakly interacting massive particles (WIMPs)

• Look for these in particle accelerators, not in space

State of flux• MACHOs and WIMPs

• No way (that is, no observations) to show which is right or even if there is a preference between the two!

• So dark matter exists, we simply do not know what it is!

The M31 and Milky Way Dance

Cosmology

Cosmology• Study of structure and evolution (time

change) of the entire universe, where and when it began, where and when it will end

• Decades ago was considered a pseudo science because you could not create experiments or observations to test…now you can

Cosmology, the history• Every culture has a cosmology!

• Greeks considered the Earth the center, cyrstalline spheres, immutable universe, …

• Look at the sacred writing of any religion!

• Change from Earth Centered to Sun Centered to Milky Way Centered to ? Centered

Science in action• Scientists do not work to prove a particular model! • Then use models to make predictions and design

experiments to test predictions

• Need to watch out for personal bias (this theory is right so make the observations fit the theory, is not science!)

• The universe may be much more than we can observe

• Dimensions we cannot sense

Types of Recent Cosmologies• Static

• Preferred by Einstein but cannot exist because of gravity

• Oscillating (limited cyclical expansion and contraction)

• Currently in an expansion cycle • Expanding • From a ‘starting point’ • Could eventually stop and contract (one cycle oscillation)

• Higher Dimension Multiverse

Consider the current evidence• Galaxies are flying away from us! • Does this make us unique?

• It would if the motion was of galaxies through space

• Again, our friend Einstein

• Space can move (think of the surface of a balloon) • If space were expanding and taking the galaxies

with it, then ALL observers in the Universe would see galaxies receding! (No preferred center!)

Examples of Space ExpandingButtons on a rubber band (elastic)

Raisin Bread

But• Do not get too enamored of the examples,

they all have edges

• Our universe has no edge!

• Unlike these examples, matter in the universe can move through space

Energy of light• Recall

• E = hν, where ν is frequency, h is a constant

• As light is shifted to be red, ν decreases, and so energy decreases

• This implies the universe is cooling!

Red Shift• Traditionally denoted z • Doppler shift z = Δλ/λ • Size of universe now S0 • Size of universe when some light was emitted S1 • S0 / S1 = z + 1 • So z = 1, means the radiation was emitted when the

universe was half its current size • We are now almost eight years since the discovery of

GRB 090423 on April 29, 2009. Its z is 8.2 so it happened when the universe was about 1/10th its current size. (3rd most distant object, 13 BLyr)

Note: Model independent• Does not matter how space expanded or

contracted or both!

Okay, if we can tell the size, can we tell the age?

• Embarrassingly, the answer is no and yes!

• No, because the age is model dependent

• Yes, because from models we can compute the age

Naïve View• Run the film backwards!

• Remember Hubble’s law: v = H0d

• Remember d = vt

• So 1 = H0t

• Bonus point: Find a current estimate of the value of H0 and compute the age of the universe. (tell me your value for the Hubble constant and your value for the age of the universe) Due 05/04/2017

But, what is the implied model?• H0 is constant!

Best guess relationship of z to time

400,000 years1000

500,000 years10

800,000 years7

1.2 billion years5

3.4 billion years2

5.9 billion years1

8.7 billion years0.5

12.4 billion years0.1

13.7 billion years0

Time after startRed Shift

Questions• Should the Universe have an edge?

• Do we have a special location in the Universe?

• Why is it dark at night? • This surprisingly simple question has a

surprisingly deep answer!

Consider• All the distant galaxies are speeding away

from us • Are we at the center?

• The distribution of galaxies is the same in each direction • Are we at the center?

• Homogeneous Universe! • Isotropic Universe!

Homogeneity and no Edge Implies…• There should be at least a galaxy at each

point of the sky,

• In fact, there should be a star at each point of the sky!

• So the Sky should be bright!!

• Olber’s Paradox [But what are the assumptions!]

Down to Earth Example

The Solution to Olber’s Paradox• Clearly, there is a problem: • It is dark at night! So there must be a problem

in the assumptions or the ‘accepted’ facts

• Surprisingly, the first correct answer was given by a non-astronomer • Edgar Allan Poe!!!

• Kepler noted this and concluded that a dark sky implies a finite universe!

“Assumed” Facts• Universe is infinite • It shows no signs of a physical edge (unlike

your view of our balloon example)

• But is it infinitely old?

• Has it always been ‘transparent’?

One Edge of the Universe• Cosmic Horizon – Distance light can travel

in the Universe’s age • Space inside the Cosmic Horizon is the visible

universe

• Since the Big-Bang Theory implies a finite age, the visible universe is finite

Another Problem• For the first 300,000 to 400,000 years after

the start of the Big Bang, the universe was so dense that radiation could not travel unimpeded (recall the radiation from the center of the Sun takes 400,000 years to go 400,000 miles)

• So where is the radiation from the time when radiation could not travel unimpeded?

• This point is the ‘last scattering epoch’

merrimack college astronomy 1101 · 2017. 4. 25. · title: ast1101_19.key created date: 4/25/2017...

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