Lesson 11: Island of Stars, Distances of Galaxies and the Big Bang Theory

Question 1

Describe the Hubble eXtreme Deep Field (HXDF). What does it show? How was it done? How far away are some of the objects in the image?

Answer 1

Hubble eXtreme Deep Field (HXDF): Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away

• Every single (except for one star) light is a galaxy

Done by:
Focusing the camera on a portion of the sky, made exposure for 11 hours, grabbing more light

Distance:
13.2 billion light-years
The most distant objects are more than 95% of the way back to the Big Bang

Question 2

What is the angular size of the HXDF

Answer 2

about one-tenth (small fraction) of the angular diameter of a full moon

Question 3

What are the main types of galaxies?

List the four of them, explain what they look like, specify hot/cold

Answer 3

Spiral Galaxy
barred/unbarred, spiral arms….. blue/cooler, young population I stars, red stars in the bulge

Lenticular galaxy
(intermediate between spiral and elliptical) - has a disk like spiral but less gas…… red/hotter, old population II stars

Elliptical Galaxy
spheroidal/oval shape, no disk… red/hotter, old population II stars

Irregular Galaxy
most common, no structure/’irregular’ shape…. blue/cooler, young population I stars and old population II stars

Question 4

Why does ongoing star formation lead to a blue-white appearance?

Answer 4

short-lived stars, recently birthed so they’re extremely bright

Question 5

What is a galaxy group? What type of galaxies will you often find there?

Answer 5

Galaxy Groups
Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies per group)

Question 6

What is a galaxy cluster? What type of galaxies will you often find there?

Answer 6

Elliptical galaxies
are much more common in huge clusters of galaxies ( hundreds to thousands of galaxies)

Question 7

What is the Sun’s nearest star and its distance away?

Answer 7

The next closest star is Proxima Centauri
4.3 light-years aways

Question 8

What is the ‘Local Group?’ What do you find there?

Answer 8

The Local Group
The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of about 10 million light-years

The two largest members are the Andromeda Galaxy and the Milky Way, which are both spiral galaxies, roughly the same mass

• There are at least 80 (most of which are dwarf galaxies)

Question 9

What do we expect to see around the Milky Way?

two things

Answer 9

We should see:

• Many stellar streams
• Many surviving satellites

Question 10

What is Sagittarius dwarf?

Answer 10

an elliptical loop-shaped galaxy
(small galaxies composed of a few billion stars)

Question 11

What are the Large and Small Magellanic Clouds?

Answer 11

irregular galaxies, cooler, filled with young and old stars, no defined shape

Question 12

What is the parallax distance method?

Answer 12

measure distances to nearby stars

Question 13

How can we use apparent brightness and luminosity to calculate distance?

state the formula

Answer 13

Distance = √1 Luminosity / 4π brightness

Question 14

What is the principle behind the ‘standard candle’ concept?

Answer 14

A standard candle : is something that we already know the Luminosity of
Therefore, we can use its apparent brightness to calculate its distance.

Think of streetlamps- they all have the same Luminosity
* The closet one appears the appears the brightest
* The furthest ones appear the less bright

Question 15

What are Cepheid Variables? Why do Cepheid Variable stars change their brightness? (how do they work)

Answer 15

Cepheids: luminous variable stars which pulsate rapidly (at intervals of days to weeks)

A Cepheid has a highly opaque surface - its outward photons push the surface outwards, which temporarily expands the star and reduces its opacity
Once it relives the pressure, it then shrinks back down

• They are physically getting bigger and smaller through time

Question 16

Why can we Cepheid Variables as standard candles?

two things

Answer 16

First, their luminosity is quite high (the most luminous Cepheids are 40,000 times more luminous than the Sun)

• already have a pre-determined luminosity

Second, can be seen to large distances and are further away (helpful for oberserving far away galaxies)

Question 17

Where are Cepheid Variable stars found on the H-R diagram? Why does that help us in measuring large distances?

Answer 17

Located on the instability strip, just left the main sequence are going to be red giants very soon
* Most of them are brighter than the sun

*This means they can be seen in other galaxies! *
due to their high Luminosity

Question 18

How do you use the Period-Luminosity Relationship to measure the distance to a Cepheid Variable star?

Answer 18

Knowing the period (easily measurable) of a Cepheid Variable will immediately tell you its Luminosity!

(distance formula)

Question 19

What was so puzzling about ‘spiral nebulae’ to astronomers in the late 1800s early 1900s?

Answer 19

Before Hubble, some scientists argued that ‘spiral nebulae’ were entire galaxies like our Milky Way, whereas other scientists maintained they were smaller collections of stars within the Milky Way
* The debate remained unsettled until someone finally measured the distance of spiral nebulae

Question 20

How did Hubble solve the ‘spiral nebulae’ puzzle? How did Hubble know that galaxies that are further away are moving away from us
faster?

Answer 20

Hubble found a Cepheid variable INSIDE the ‘spiral nebula’ Andromeda

Then
○ Measured distance to Andromeda
○ Measured distance to other galaxies and figured out most of their spectra **is red-shift / moving away **

Question 21

What is Hubble’s Law?

Answer 21

Hubble’s Law
The further away the galaxy is, the faster its moving away from us (the more red-shifted it is)

Question 22

What is Hubble’s Law? (describe an equation), What is Hubble’s Constant (H0)? How does Hubble’s Law help us measure the age of the Universe?

Answer 22

Hubble’s law:
velocity = H0 × distance

Hubble’s Constant
(rise/run = H0): the rate of the Universe’s expansion

• 1/H0 can tell you the age of the universe

Question 23

Explain the the “Balloon universe” analogy

Answer 23

The expansion rate appears to be the same everywhere in space. The universe has no centre and no edge — much like the surface of a balloon

• describes how the universe is expanding, and making all galaxies move away from all other galaxies

Question 24

White Dwarf Supernovae can also be considered standard candles, why is that?

Answer 24

White dwarf Supernovae are intrinsically brighter than Cepheid Variables, they can delve deeper into the universe, to when it was much younger

• The apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light- years)

Question 25

Explain how we use a multiple different methods of distance measurements in a ‘chain of interdependent techniques’

(4 techniques in order)

Answer 25

1. Parallax - only for closer objects
2. Cepheids - can measure for nearby galaxies
3. Hubble’s law - redshift technique for distant galaxies
4. White dwarf supernova - measure their brightness for distant galaxies

Question 26

What is the Big Bang Theory? What two things does it predict we should be able to observe about our Universe?

Answer 26

Big Bang Theory: how astronomers explain the way the universe began

It predicts that:
(1) The density of the universe in the past must have been higher (dense - how compact things were)

(2) The ‘glow’ from early universe should still be visible
If you look any direction in the universe, you see microwave light that was formed when the universe was created

Question 27

define Critical density

Answer 27

Critical density—density required for gravity to halt the inital expansion momentum

• If the actual density is higher than this critical density, then the expansion will ultimately reverse and the universe will begin to contract.
• If the actual density is lower, then the universe will expand forever.

Question 28

What is the Cosmic Microwave Background?

Answer 28

Cosmic Microwave Background: leftover radiation from the Big Bang or the time when the universe began

Question 29

What are the four possible ‘fates’ of the universe, based on our understanding of gravity (Einstein’s Theory of General Relativity)?

Answer 29

Closed universe - “Big crunch”, universe is collapsing on itself
* Curve 1: the actual density of the universe is higher than the critical density and there is no dark energy. This universe will stop expanding at some time in the future and begin contracting.

Open universe - continuous slow expansion , “Big Chill”
* Curve 2: the density of the universe is less than the critical density, gravity is never important enough to stop the expansion, and so the universe expands forever.

Critical-density universe, “Big Halt”
Curve 3: the universe will eventually come to a halt in expanding

Universe with dark energy, “Big Rip”
Curve 4: the rate of the expansion will increase with time, and the expansion will continue at an ever-faster rate.
* The Universe that we are predicted to live in

Question 30

What are the values 𝝆, 𝝆crit, and Λ, conceptually?

Answer 30

𝝆 - density of the universe

𝝆crit - critical density - inward gravity

Λ - cosmological constant - an (unknown) outward push

Question 31

What did the supernova project conclude about the most likely fate of our universe?

Answer 31

the results: Hubble’s Constant (H0) was lower in the past, therefore, the Universe is expanding at an accelerating rate, towards a Big Rip

Question 32

What is Dark Energy?

Answer 32

a cosmological constant, that is driving our universe to expand more and more

Question 33

What does it mean to say ‘cosmic tug of war’? What two forces are participating in this tug of war?

Answer 33

The Cosmic Tug of War
the force of gravity and the expansion of the universe