Exam 3 Flashcards
Is there a minimum mass to a planet? A maximum mass?
Both
What provides pressure support for the Sun?
Nuclear fusion
Patterns in the stars tell us their
life stage
The mass of a star controls its rate of
core nuclear fusion
High-mass stars
live fast and die young
What do the bright blue stars in the Pleides tell us about their age?
They are high mass and young
Supernova
Stars born with >10x the Sun’s mass end their lives explosively
What mass of host star would be most likely to have a
planet with life?
0.3 Msun
“Planetary Nebula”
Eventually, stars shed their outer envelopes and only core remains
In a few billion years, the radius of the Sun will
increase
hydrostatic equilibrium
outward pressure must balance inward pull of gravity
Pressure in Sun comes from
H [hydrogen] to He [helium] fusion
4 H atoms
have a little less mass than 1 He atom, and that mass difference was converted directly into energy from fusion
Patterns in temperature vs. luminosity (intrinsic brightness) reveal
the life cycles of stars, where mass is the factor controls how fast star ages
When low-mass stars like the Sun run out of fuel for fusion its radius eventually expands into
red giant
When it is a giant, The Sun will have a radius extending to
Earth’s orbit
Earth is likely to be engulfed by the Sun then, in about
5-6 billion years
Eventually only the core (60% of Sun’s mass) will be left:
white dwarf
White dwarf
has about 60% the Sun’s mass packed into a volume the size of the Earth (1 teaspoon of white dwarf weighs more than a car)
All stars less than 8x the Sun’s mass
first become red giants than end their lives as white dwarfs when they run out of core H & He
In a few billion years, the surface temperature of Earth will
increase
What is the remnant white dwarf mostly composed of?
Carbon/Oxygen
Massive stars have
massive core temperatures
Heavier element fusion requires
hotter core temperatures
Why do we not expect the Sun to fuse Si à Fe
(silicon to iron) in its core?
The Sun is not massive enough to get core
temperatures hot enough for Si fusion.
Each advanced fusion reaction generates
less energy
Iron fusion
is the end of the line for massive stars.
What happens when a massive star runs out of pressure support?
It implodes and then explodes
Crab Pulsar
is the remnant of a supernova that went off in our
galaxy in 1054 AD
- at the center: a rapidly spinning neutron star
Pulsar
a rapidly spinning neutron star
- Astronomers used pulsar as clock to see wobble from planets: this is how the very first exoplanets were discovered in 1992
What are the pulses in a pulsar?
the rotation of the neutron star
It is unclear if pulsar planets are first- or second-generation.
(1) Are they the small cores of giant planets that survived
the supernova explosion?
(2) Were they re-generated from the leftover debris from the exploded star?
What has happened in this history of a planet around a pulsar?
It’s host star exploded
What is a black hole?
Something so dense that not even light can escape
- the escape velocity exceeds the speed of light
Special Relativity
Time and space are not absolute:
- They are relative and depend on your perspective (and motion)
General Relativity
Gravity is explained by mass bending space
- More massive objects bend space more
- orbits are straight-line motion curved by the shape of space
What happens to a planet’s orbit if the star it was orbiting disappears?
It keeps going in a straight line.
If a star (or any mass) passes in front of a background star,
it can act as a lens to the background star’s light.
If that star has a planet,
it will also lens the background star!
Microlensing occurs
when any mass passes in front of a background light source!
Could we find a black hole from gravitational microlensing?
Yes
What happens if a black hole passes in front of a star?
A blip
Dense stars like white dwarfs can actually create
“self-lensing binaries” when passing in front of another star
What is at the heart of Einstein’s theory of general relativity?
Mass bends space
Are we guaranteed to see the star that causes the microlensing event?
No
Can we follow up on any planets found after the microlensing event
No
In 2022, what method would you expect finds
the newest exoplanets?
Transits
Can hot Jupiters have formed where we find them?
No
- We think hot Jupiters migrated in close to their host
star by drag in the disk in which they formed.
- They formed far out and moved in over time
What is causing the gaps in the disks
Mass from young planets will shape the disk – mass interacts with mass to create resonances and thus gaps!
“Nice Model”
explores the evolution of orbits in our Solar System
After the orbits of the giant planets were rearranged
Our solar system likely went through instability 3.9 billion years ago (Late Heavy Bombardment)
Orbital Resonances
can transfer energy and can often stabilize orbits: Galilean moons of Jupiter & TRAPPIST-1’s seven-planet chain
What has today’s lecture said about the
orbits of giant planets?
Planetary orbits can be influenced by gravitational interactions with other planets or the disk they form in.
How many planets did we know about in 1785?
7
Uranus
discovered in 1781 by William Herschel, noting a
relatively bright object wandering against background stars.
Neptune
discovered with a telescope in 1846,
- predictions of the orbit of Uranus using Kepler’s
laws were starting to fail. Something yet undiscovered was perturbing the orbit of Uranus
- gravitational influence was inferred before it was
directly discovered
“Planet Nine”
In 2014, astronomers Konstantin Batygin and Michael
Brown suggested a ninth planet, yet undiscovered, may
exist in our solar system:
- Trans-Neptunian Objects (friends of Pluto, but futher out) have strange orbits that are most simply explained by a new planet not yet discovered
- might have >5x Earth’s mass, 400-800 au
How many inhabited worlds do we know of in our Solar System?
One
The Fermi Paradox: “Where is everybody?”
- Hundreds of billions of stars in our Galaxy, many are likely habitable
- Why do we have no evidence of past civilizations visiting Earth?
Avi Loeb
white, Harvard professor who insists aliens have visited Earth
‘Oumuamua
likely the first object with interstellar origins passing through our Solar System that humans have ever identified
- It accelerated going out of the solar system but did not outgas, so isn’t a comet
- Avi Loeb has claimed that this could be a thin solar sail built by extraterrestrial life, though few astronomers so far agree
How could we make contact?
SETI has been listening for nearly 60 years around >1000 stars
Arecibo Message
We have announced our presence by sending out a radio signal towards a cluster of stars roughly 25,000 lightyears away
- series of binary 1s & 0s, an extremely dense communique to inform about our chemistry, DNA, size & shape, Solar System, etc.
Drake Equation
N is the number of civilizations with which communication is possible
(Still major uncertainties in some factors, so our expectation of the number of communicative civilizations in our Galaxy is still completely unknown)
what is the chance that a random star in our Galaxy has a civilization that can communicate?
If there are 200 billion stars
in our Galaxy, then 15,750/200,000,000,000 = 0.00000008: there is a roughly
0.000008% chance that a random star in our Galaxy has a civilization that can communicate
Fp
fraction of stars with planets: 1
This estimate was a large legacy of the Kepler mission, which found that on average roughly every star has 1 planet orbiting with a period <300 days.
Ne
Number of planets that can develop life per star with planets: 0.15
A sensible value comes from the estimate from the Kepler mission that roughly 15% of stars host a planet in the habitable zone
Fl
fraction of potentially habitable planets that develop life: 0.1/10%
Fi
fraction of planets that develop intelligent life: 0.01 (differs)
Fc
fraction of intelligent life that can develop interstellar communication
L
lifetime of a communicative civilization
(value was perceived by scientists a bit lower during the Cold War when humans were near the brink of nuclear self-annihilation)
