Astronomy Test 3

What is hydrostatic equilibrium

a balance of gravity pulling in and pressure from heat pushing out

Luminosity

total energy radiated by a star (total brightness)

* total brightness

What is nuclear fusion

joining of 2 nuclei together to form different one

* requires high temp, and density to overcome electrical repulsion

A star becomes stable when what?

When the outward forces of expansion from the energy released in nuclear fusion reactions balance the inward forces of gravity

Very massive stars

are rare

Low-mass stars

are common

We are “star stuff” because

The elements necessary for life we made in stars

The sun sits about where?

About 2/3 of the way from the center to the edge of the disk

The sun revolves around the center of the galaxy about once every

250 million years

Origin of stars

Stars are born in Nebulas which are swirling clouds of hydrogen gas

The matter between the stars are collectively termed as interstellar medium. It’s made out of 2 components

Gas (75% Hydrogen, 25% helium) a little of other gases and dust

Any interstellar cloud of gas and dust is called a

Nebula (plural Nebulae)

Evidence of Nebulae

• spectral lines

• reddening of stars

Stars form in where?

Dark cloudy dusty gas in interstellar space

Interstellar medium

The gas between the stars

Emission Nebula

A nebula with the characteristic emission line spectrum of a hot, thin gas

The vast amounts of UV radiation emitted by the close by Hot, type O or type B stars are absorbed by

The hydrogen atoms in Nebulae

Emission nebular are referred to as

H II regions

Dark nebula

A nebula so opaque that it blocks visible light that are emitted from stars behind the nebula

Reflection nebula

Doesn’t produce its own light like emission nebulae, but scatters star light

• scattering is due to the dust grains

• lower concentration of dust grains than dark nebulae

• scattering gives rise to blue color

Interstellar extinction

The intensity of star light is reduced as light passes through the interstellar medium

Interstellar reddening

When a light from a star pass through interstellar medium, dust particles absorb or scatter blue light allowing red light to pass through (like a sunset on earth)

Long-wavelength infrared light passes through a cloud more easily than

Visible light

Observations of infrared light reveal stars

On the other side of the cloud

Giant molecular clouds

In certain cold regions of interstellar space, atoms combine to form molecules

Stars are born in

Molecular clouds consisting mostly of hydrogen molecules

Stars from in places where gravity can

Overcome thermal pressure in a cloud

Star-forming clouds emit infrared light because

The heat generated as stars form

Infrared observations can directly detect

Dust in the interstellar medium

Some molecules in the cold gas emit in the

Infrared- infrared observations can detect very cold clouds of gas

The birth and life of any star can be described as a battle between two forces

Gravity vs. internal pressure

Gravity always wants to what

collapse a star

internal pressure wants to what?

hold up a star

Newton’s law of gravity

• the amount of gravitational force depends on the mass

• gravitational potential energy is turned into heat as a star collapses

In the densest clouds, hydrogen can exist as what?

As molecules (H2) rather than atoms

* These clouds are called **Molecular clouds**

Formation of stars form the interstellar medium- **An interstellar molecular cloud**

• star formation begins when part of the interstellar molecular cloud contracts under its own gravitational attraction- denser regions in the clouds are favorable for star formation

• the gravitational collapse overwhelms the presssure-colder regions are more favorable since they aren’t low-pressure regions

• these cold dense regions of clouds collapse under its own weight to form clumps, future stars

Formation of stars from the interstellar medium- **Contracting Cloud**

Star formation is triggered when a sufficiently massive pocket of gas is squeezed by some external event

• material flowing out of protostars cause shock waves that trigger regions nearby to collapse

• a supernova explosion of a dying star can compress the surrounding gas triggering a collapse

A star-forming cloud colliding with a shock wave can

be compressed and break into fragments

* some of these fragments can become dense enough to collapse under gravity and form stars

Fragmentation of a cloud- gravity within a contracting gas cloud becomes stronger as

the gas becomes denser

fragmentation of a cloud- gravity can therefore overcome pressure in smaller pieces of cloud, causing it to break apart into

multiple fragments, each of which may go on to form a star

Each lump of the cloud in which gravity can overcome pressure can

go on to become a star

A large cloud can make

a whole cluster of stars

The dense opaque region at the center is called a

protostar- an embryonic object at the dawn of star birth

observing the infrared light from a star cloud can reveal

the newborn star embedded inside it

Solar-system formation is a good example of

star birth

the rotation speed of the cloud from which a star forms increases as

the cloud contracts

rotation of a contracting cloud speeds up for the same reason

a skater speeds up as she pulls in her arms

what is flattening

collisions between particles in the cloud cause it to flatten into a disk

formation of jets

jets are observed coming from the centers of disks around protostars

from protostar to main sequence- protostars looks starlike after the surrounding gas is blown away, but its thermal energy comes from

gravitational contraction, not fusion

from protostar to main sequence- contraction must continue until the core

becomes hot enough for nuclear fusion (10,000,000 K)

from protostar to main sequence- contraction stops when the energy released by core fusion balances energy radiated from the surface- the star is now

a main-sequence star

when thermonuclear reactions start at the center of a protostar

we say a new star is born

summary of star birth

1. gravitational causes gas clouds to shrink and fragment

2. core of shrinking cloud heats up

3. when ore gets hot enough, fusion begins and stops the shrinking

4. new star achieves long-lasting state of balance

stages of star formation on the H-R diagram- life track illustrates star’s surface temperature and

luminosity at different moments in time

evidence of star formation

• bipolar flow from young stars

• star forming regions

• young star clusters

Summary: How do stars form?

• stars are born in cold, relatively dense molecular clouds

• as a cloud fragment collapses under gravity, it becomes a protostar surrounded by a spinning disk of gas

• the protostar may also fire jets of matter outward along its poles

• protostars rotate rapidly, and some may spin so fast that they split to form close binary systems

Which range of the electromagnetic spectrum is best for observing newly formed stars or inside of the clouds where stars are born?

Infrared

The birth of a star is designated with what?

with the start of hydrogen to helium, fusion in all stars

Star starts dying when

hydrogen to helium fusion stops

When is a star born?

Hydrogen fusion proton-proton chain starts at the center of a protostar, we say a new star is born

Cluster formation

1. molecular cloud

2. hot gas

3. young cluster

   1. old cluster

There are 2 kinds of clusters

Open and globular

Open clusters

* young clusters of a few 1000 stars
* blue main-sequence and few giants

Testing stellar evolution- the problem

• stellar evolution happens on billion-year time scales

• astronomers only live for a few 10s of years

Testing stellar evolution- the solution

Make H-R diagrams of star clusters with a wide range of ages

What are star clusters

group of 100s to 1000s of stars

• are at the same distance so it’s easy to measure

• have the same age

• have the same chemical composition

• have a wide range of stellar masses

Each cluster provides a snapshot of

what stars of different masses look like at the sane age and composition

Each star cluster freeze-frames and makes what

a visible moment in stellar evolution

* the differences you see among stars in one cluster must arise from differences in their masses

The main sequence on the H-R diagram is what?

A mass sequence

• massive stars are HOTTER and BRIGHTER

• low-mass stars are COOLER and FAINTER

Main sequence lifetime depends on

Mass

• massive stars have SHORT main sequence lifetimes

• low-mass stars have LONG main sequence lifetimes

Low-mass stars take longer to do what?

to form

Clusters in the H-R diagram indicate what?

Age

• all stars arrive on the main sequence are about the same age

• the cluster is as old as the most luminous (massive) star left on the main sequence

All main sequence stars to the left have already used up their what?

Their H fuel and are gone

The position of the hottest brightest star on a cluster’s main sequence is called the

Main sequence turnoff point

* main sequence turnoff point of a cluster tells us its age

As clusters age they start with high-mass stars on the main sequence and

low mass stars still approaching

as a cluster ages, high mass run out of

hydrogen in their cores first, evolving off into supergiants

as a cluster ages, lower mass stars run out of

hydrogen in their cores, they too evolve off

as a cluster ages, stars peel off the main sequence

from the top (high mass end) down as the cluster ages

by comparing clusters of different ages, you can

visualize how stars evolve- almost as if you were watching a film of a star cluster evolving over billions of years

Main sequence turn off

point where the main sequence “turns off” towards giant stars

• as cluster ages, the stars at the turn off are lower mass

• low mass stars have redder colors

Color of the turn off is an indicator of what?

of the cluster age

* older clusters have ==redder== turn off points

A star remains on the main sequence as long as it

can fuse hydrogen into helium into its core

The mass of a main sequence star determines its

core pressure and temperature

stars of a higher mass have higher

core temperature and more rapid fusion, making those stars both more luminous and shorter-lived

Stars of lower mass have cooler cores and slower fusion rates, giving them

smaller luminosities and longer lifetimes

The differences in whether a star become a red giant or white dwarf is determined by

the mass of the star

The structure of an average mature star- what reactions occur in the core releasing gamma and X ray radiation?

hydrogen fusion

What radiation moves through the radiation zone from particle to particle eventually heating gases at the bottom of the convection zone?

Gamma and X-ray

What carries energy to the surface where it’s emitted to space as visible light, ultraviolet radiation, and infrared radiation?

Convection cells

What happens when a star can no longer fuse hydrogen to helium in its core?

Core shrinks and heats up

what happens to a star’s inert helium core starts to shrink?

hydrogen fuses in shell around core

as the star contracts, H beings fusing very rapidly to

He in a shell around the core

• luminosity increases because the fusion rate is higher

• size increases: red giant phase

Life history of a sunlike star- Helium fusion requires

higher temperature than hydrogen fusion

Life history of a sunlike star- Helium fusion combines

3 He nuclei to make a carbon

Core shrinkage as a star ages- as fuel is exhausted, outward pressure in core drops and gravity

compresses it

The expansion of a star to giant or supergiant size cools the star’s

outer layers (the star moves towards the upper right in the H-R diagram

on the H-R diagram the top left area represents

hot, bright stars

on the H-R diagram the top right represents

cool, bright stars

on the H-R diagram the bottom left represents

hot, dim stars