Introduction to Astronomy
Lecture10:
The next big thing(s)
– some current and future hot topics in
astronomy
Presented by
Dr Helen Johnston
School of Physics
Spring 2015
The University of Sydney
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In tonight’s lecture
• A tale of two stars
– the peculiar X-ray binary Cir X-1
• Exoplanets
– the amazing success of the Kepler mission
• The next big things in Astronomy
– REALLY big telescopes
– upcoming space missions
– astronomy without light: particle astrophysics and gravity waves
• To infinity and beyond!
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A tale of two stars
– the peculiar X-ray binary Cir X-1
The peculiar X-ray binary Cir X-1
Cir X-1 is an X-ray binary which defies easy categorisation.
– Orbital period 16.6 d; P/Ṗ = 3000 y
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The peculiar X-ray binary Cir X-1
Cir X-1 is an X-ray binary which defies easy categorisation.
– Orbital period 16.6 d; P/Ṗ = 3000 y
– X-ray light curve is very asymmetric, with dips (eclipses?) at
phase 0
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The peculiar X-ray binary Cir X-1
Cir X-1 is an X-ray binary which defies easy categorisation.
– Orbital period 16.6 d; P/Ṗ = 3000 y
– X-ray intensity has varied by >100 over 40 years
– X-ray bursts seen in two epochs neutron star
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Properties
HMXB (young)
LMXB (old)
Donor
O–B (> 5M⊙)
K–M
Optical spectrum
Star-like
Re-processed
Accretion disk
no (small)
yes
Orbital period
1–100d
0.1–10d
X-ray eclipses
common
rare
B-field
high (~ 1012 G)
low (~ 107–108 G)
X-ray pulsations
common
rare
X-ray bursts
no
common
QPOs
rare
common
Jets
no
yes
–
–
–
Optical counterpart is faint red star with strong Hα
Nature of companion star unknown: source is near the Galactic centre, in a crowded
field with AV ~ 9–12
Is the companion faint or bright? More reddening can hide brighter star.
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No stellar features! Every absorption feature is a DIB.
Derive reddening: from DIBs, AV = 7.6 ± 0.6 mag
From Balmer decrement, AV > 9.1 mag
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GMOS spectrum with
features identified
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GMOS acquisition images
1.2 mag drop in 4 days
– suggests re-processed light
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Data from Gemini acquisition images from 2013
Data from Moneti (1992) in blue
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What if light variation comes from heated face of companion?
Eccentric orbit more X-ray heating when stars are close.
Assume the companion star fills its Roche lobe at periastron of an eccentric
orbit (eccentricity e):
R1 = RL,peri = a(1
0.49q 2/3
e)
0.6q 2/3 + ln(1 + q 1/3 )
M1
where q =
M2
Then size of companion is uniquely
determined by shape of orbit
(q and e).
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Fit to light curve for e = 0.4 (insensitive to q):
Low temperatures preferred.
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Meanwhile...
4 arcmin
X-ray/radio remnant (Heinz et al. 2013)
age t < 5600 y
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X-ray light echo (Heinz et al. 2015)
distance d = 9.4 ± 1.0 kpc
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Constraints on companion star
– Known brightness + reddening + distance
MV =
2.2
– Time since supernova + lifetime of pre-supernova star
7
⌧  4 ⇥ 10 y
‒ Size of star = size of Roche lobe + mass of star
h⇢i < 0.003 g cm
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Padova PARSEC isochrones
No stars satisfy criteria:
– stars which are low enough
density are too bright, unless
they are low mass, in which
case they don’t satisfy the
age criterion.
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A companion star heated by the supernova
Companion star was impacted by supernova explosion < 5000 y ago.
Ejecta impact can expand and heat companion; returns to MS
on thermal timescale (~ several thousand years).
We may be seeing the companion star to
Cir X-1 in this puffed-up state.
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Properties
HMXB (young)
LMXB (old)
Donor
O–B (> 5M⊙)
K–M
Optical spectrum
Star-like
Re-processed
Accretion disk
no (small)
yes
Orbital period
1–100d
0.1–10d
X-ray eclipses
common
rare
B-field
high (~ 1012 G)
low (~ 107–108 G)
X-ray pulsations
common
rare
X-ray bursts
no
common
QPOs
rare
common
Jets
no
yes
new ways of looking at the Universe
Exoplanets and Kepler
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The Kepler mission to find transiting
planets was launched in March 2009. It
is designed to observe 155,000 stars in
a single field in Cygnus, observing
continuously (every 30 min) for 3.5
years.
To detect the transit of an Earth-like
planet, it needs to detect brightness
changes of 1/10,000 when an Earthsized planet on an Earth-like orbit
makes a ∼12-hour passage in front of a
Sun-sized star.
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Kepler is in a heliocentric, Earth-trailing orbit, falling gradually further
behind the Earth. It is pointing to a region in the Orion arm, in the
direction of the Sun’s motion around the Galaxy.
First
images from Kepler, showing the whole field of view,
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and an open cluster and a star with a transiting planet
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From its first 10 days of commissioning data, Kepler detected a
previously known giant transiting exoplanet, HAT-P-7b
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Since then, Kepler has spent four years finding planets. Planetary
candidates are only confirmed when large ground-based telescopes
have detected the radial velocity variations
due to the planet. There are so many
candidate planets that these confirming
observations are now the limiting step.
So far, Kepler has discovered 1030
confirmed planets.
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The planet verification bottleneck
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Here are some of Kepler’s most exciting planets:
• In 2011, the Kepler team announced the first discovery of a
circumbinary planet – a planet orbiting two stars. The two orbiting
stars regularly eclipse each other; the planet also transits, each star,
and Kepler data from these planetary transits allowed the size,
density and mass of the planet to be extremely well determined.
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Artist’s impression of Kepler-16b,
the “Tatooine planet”
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Primary
eclipse
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Secondary
eclipse
Light-curve of Kepler-16b , showing primary and
secondary eclipses, as well as the transits of the planet
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across each star.
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Bird’s eye view of the Kepler-16b system. The planet, which is 1/3 the mass of
Jupiter, orbits its star at a distance comparable to that of Venus in our own
solar system, but is actually cold, as both stars are cooler than our Sun.
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• The same year, the Kepler team announced the discovery of a
system of six low-mass planets transiting Kepler-11.
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All six planets have orbits smaller than Venus, and five of the six have
orbits smaller than Mercury's. The sizes are between 2 and 5 Earth
radii.
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The current record number of planets is seven, around Kepler-90 (KOI 351)
Comparison of
Kepler’s multipleplanet systems with
the Solar System
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(top)
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Ten circumbinary planets have now been discovered. The latest,
around Kepler-453, has a Neptune-mass planet in a 240 day orbit
around a binary with an
orbital period of 27 days.
Artist's impression of the Kepler-453 system
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(image
by Mark Garlick)
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Forming circumbinary planets is very difficult to explain: the gravity
perturbations from the two stars on a
circumbinary disk would have led to
many destructive collisions.
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Artist's impression of Kepler-34b, a gas-giant planet that
orbits a double-star system. Its two suns are both yellow
G-type stars that swing around each other every 28 days.
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The planet circles them both in 289 days.
And planets orbiting one star in a binary can find themselves bouncing
from star to star.
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The Holy Grail has been to find a planet located in the habitable
zone, where it should be possible for Earth-like conditions (particularly
liquid water) to prevail.
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Kepler has found many planets in the habitable zone. In July 2015
they announced the discovery of the first Earth-sized planet in the
habitable zone of a
star like ours.
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In May 2013, the second of Kepler’s four reaction wheels failed.
Without the ability to maintain its orientation, the spacecraft was no
longer able to point precisely enough, thus terminating the main
mission.
In May 2014, an extension mission called K2 was approved. This uses
the solar wind to help stabilise the spacecraft, recovering
pointing stabiity.
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Kepler will stare at target fields in the ecliptic for about 75 days,
before the spacecraft has to be rotated again.
This means completely different fields will be observed each time.
Kepler is currently observing its 5th field, C5.
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There is a “Citizen Science” project associated with Kepler, where
members of the public identify transit events in the light curves to identify
planets that the computer algorithms might miss. The first person to flag a
potential transit gets credit for the discovery, and is offered authorship on
the paper.
At least 60 new candidates have
been identified and two confirmed
planets, including the first known
planet in a quadruple star system.
A family portrait of the PH-1 planetary system: The newly
discovered planet is depicted in this artist’s rendition
transiting the larger of the two eclipsing stars it orbits. Off
in the distance, well beyond the planet orbit, resides a
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second pair of stars bound to the planetary system.
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new ways of looking at the Universe
Telescopes: the next generation
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Very Big Telescopes
A whole new generation of enormous telescopes, both optical and
radio, is currently under construction.
Here are a few of the telescopes which will be coming online in the next
decade.
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The Square Kilometre Array (SKA) is the world’s largest radio telescope.
The aim is to link multiple dishes with a total collecting area of one
square kilometre, making it 50 times more sensitive than any other radio
telescope.
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The SKA is a global collaboration of 10 countries, including Australia.
The telescope will be shared between Africa and Australia, with Africa
hosting the high frequency array and Australia hosting the lowfrequency array. The superb sensitivity of the SKA will aim to provide
answers to fundamental questions about the origin and evolution of the
Universe. Construction is scheduled to begin in 2018, with first
observations in 2020.
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Next week
Extremely large telescopes
There are currently three projects underway to build optical telescopes
with diameters > 20m. ELTs are planned to be able to see back to the
very early universe, and to detect Earth-like planets around other stars.
Construction costs are ~ $1 billion for each.
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The Giant Magellan Telescope (GMT) will consist of seven 8.4-m
circular mirrors, and will be located in Chile. Four mirrors have been
cast, and will be completed by 2021. Australia is a member of the
consortium building the GMT.
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The Thirty Meter Telescope (TMT) is a 30-m telescope, made up of 492
hexagonal segments, each 1.4 m in diameter. It will be located on
Mauna Kea. Construction has been halted by protests by Hawaiians.
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The European Extremely Large Telescope (E-ELT) is a 39.3-m telescope
consisting of 798 hexagonal segments, each 1.45 meters across but
only 50 mm thick. It will be located in Chile. The design has been
approved, and the site has been selected; operation will begin in
2024.
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The James Webb Space Telescope (JWST) is a 6.5-m telescope built as a
successor to Hubble. It will observe from long-wavelength visible to the
mid-infrared, and will be
located near the Earth–Sun L2
point. It is scheduled for launch
in 2018.
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The Gaia mission to measure parallax distances to a billion stars has
been running since early 2014. The first catalogue of results will be
released next year.
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Astronomy without light
– new ways of looking at the Universe
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Back in lecture 1, I told you that astronomers could use only
electromagnetic radiation to learn about the Universe.
There are actually several experiments going on to observe the
Universe in other ways.
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Neutrino astronomy
Many astrophysical reactions produce neutrinos – sub-atomic particles
which have no mass or charge – in particular, the nuclear fusion
reactions that power our Sun and other stars. Detecting neutrinos allows
us to better understand these reactions.
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Unfortunately, neutrinos interact hardly
at all with matter, so they are very
difficult to detect.
Raymond Davis built the first neutrino
detector in the 1960s. The detector
consisted of 400,000 litres of cleaning
fluid (C2Cl4) one mile underground in
an old mine.
When a chlorine atom captures a
neutrino, it is turned into a radioactive
isotope of argon. A few dozen events
are expected every month.
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The results of the experiment were astonishing. Davis detected only
about a third of the expected number of neutrinos!
In his Nobel Prize lecture (2002), Raymond Davis says
“The solar neutrino problem lasted from 1967–2001. Over this
period neither the measured flux nor the predicted flux changed
significantly. I never found anything wrong with my experiment. John
Bahcall never found anything wrong with the standard solar model.”
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There were several possible explanations for this.
1. Nuclear fusion is not taking place in the Sun (but other evidence
suggests it is)
2. The experiment was not calibrated properly (but several other
experiments have detected the same result)
3. The rate of fusion inside the Sun is lower than we thought (but we
know how much energy has to come out!)
4. Something is happening to some of the neutrinos on the way so we
can’t detect them.
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In the late 1990s a new detector
called Super-Kamiokande was built in
Japan, using 50,000 tonnes of pure
water and 11,200 photomultiplier
tubes (PMTs) in a 40m high x 40m
diameter cylinder.
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Super-Kamiokande detects neutrinos using Cherenkov radiation. If a
charged particle travelling very close to the speed of light travels into a
medium where the speed of light is lower (e.g. water), it can find itself
travelling faster than (local) light speed (e.g. in water vlight = 0.75c).
This produces a bow shock of light ahead of the particle, called
Cherenkov radiation. The advantage is that you can measure the time of
arrival and direction of the neutrino.
Fuel assemblies cool in a water pond at the French nuclear
complex at La Hague. The blue light is generated by
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Cherenkov radiation,
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In 1998 a Super-Kamiokande team announced they had detected
evidence of neutrinos oscillating between different varieties, only one
of which – the electron neutrino – can be detected.
This not only explains the results of the solar neutrino experiment, but
also implies that neutrinos have mass.
The 2015 Nobel Prize in Physics was awarded to Takaaki Kajita (of
the Super-Kamiokande Collaboration) and Arthur McDonald (of the
Sudbury Neutrino Observatory in Canada) for their discovery of
neutrino oscillations.
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So far, the only extra-solar system object detected by neutrino
observatories was SN 1987a. Two neutrino observatories – the
Kamiokande experiment in Japan and the IMB experiment in Ohio –
both detected a burst of neutrinos associated with the supernova: 12
and 8 events respectively, observed some hours before the optical
supernova was spotted. Of course, the connection with the supernova
was not realised until after the optical discovery.
One of the neutrino events from the IMB detector. The neutrino
produced a flash of light, which was detected by several photomultiplier tubes: by noting which PMTs responded, the direction
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and
energy
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Several of the next generation of neutrino detectors are starting to
come online, with much larger detector volumes (~1 km3), all detecting
Cherenkov radiation from neutrino interactions:
• NESTOR and ANTARES: towers of
PMTs anchored to floor of the
Mediterranean Sea
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• IceCube: strings of PMTs lowered into Antarctic ice. Holes are drilled
2 km under the ice to where the ice is clear. The ice acts as a
detector a cubic kilometre in size.
IceCube has detected neutrinos from outside
the Solar System, but has not yet identified
the source.
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Gravitational waves
Remember that in lecture 5, we learned that general relativity predicts
the existence of gravitational radiation: fluctuations in space-time which
propagate as a wave. The most intense sources of gravity waves will
come from e.g. coalescing black
holes.
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Gravitational waves are detected by detecting a change in lengths,
e.g. the change in the distance between two objects.
The effect is extremely weak: the most violent event produces changes
of about 1 part in 1021. To measure this, you need to be able to
measure the change in length equal to 0.1% x diameter of a proton
over 4 km.
Detecting gravitational waves emitted by a binary pulsar
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of Sydney a distortion (image by Matthew Francis)
byUniversity
measuring
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LIGO, the the Laser Interferometer Gravitational Wave Observatory,
uses laser interferometry to measure accurate distances between test
masses 4 km apart inside vacuum pipe. A passing gravitational wave
will slightly stretch one arm as it shortens the other.
LIGO has two installations 3000 km apart to distinguish e.g. seismic
events.
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Europe has a similar installation called VIRGO: two 3 km arms using
laser interferometry, located near Pisa.
No gravitational waves have yet been detected by any observatory.
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To infinity and beyond!
• Read “Astronomy Picture of the Day” for all the best astronomy
images and news http://http://apod.nasa.gov/apod/
• Read an astronomy blog, like “Bad Astronomy”
http://www.slate.com/blogs/bad_astronomy.html or “Snapshots from
Space” http://www.planetary.org/blogs/emily-lakdawalla/
• Join a local astronomical club: see listing at the Astronomical Society of
Australia page
http://www.astronomy.org.au/ngn/engine.php?SID=1000022&AID=100136
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And, of course, attend more Continuing Education courses! Future
courses include
Lives of the Stars
A more detailed look at how stars live and die
Voyage to the Planets
A look at the solar system in the era of space exploration
plus occasional other courses, such as
Eyes on the Prize
A history of astronomical discoveries which have been awarded the Nobel Prize
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That’s all, folks!
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Further reading
• The Kepler website is at http://kepler.nasa.gov: they have loads of good pictures and animations, as well
as a live counter telling you how many planets they’ve found!
• The Citizen Science project Planet Hunters is at http://www.planethunters.org; you can help identify
transits and find new planet candidates. The site is part of the zooniverse project, which has many
participatory projects, from identifying galaxies, to exploring features on the Moon, and identifying
potential Kuiper Belt targets for New Horizons.
• There’s a nice book called “The edge of physics: A journey to earth’s extremes to unlock the secrets of the
universe” by Anil Ananthaswamy (Duckworth Overlook, 2010), in which the author travels to all sorts of
remote places to find out about some of the new big experiments on cosmology. A fun read, that gives a
good feel for some of these exciting new facilities. The author also has a website with photos from his
journeys at http://www.edgeofphysics.com.
• Fred Watson’s book “Stargazer: The life and times of the telescope” doesn’t really get as far as ELTs,
except in passing, but he does give you an excellent feel for why astronomers always want a bigger
telescope.
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Sources for images used:
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Cir X-1 starting image: from http://chandra.harvard.edu/photo/2013/cirx1/
Cir X-1 images from Johnston et al. 2015 http://adsabs.harvard.edu/abs/2015arXiv151103701J and references therein.
Chandra X-ray images: from “X-ray Binary Circinus X-1” http://www.nasa.gov/mission_pages/chandra/multimedia/xray-binary-circinus-x1.html and APOD 2015 Aug
5 http://apod.nasa.gov/apod/ap150805.html
All Kepler images are from http://kepler.nasa.gov
HAT-P-7b light curve: from Borucki et al, 2009, “Kepler’s Optical Phase Curve of the Exoplanet HAT-P-7b”,
http://adsabs.harvard.edu/abs/2009Sci...325..709B; animation from http://kepler.nasa.gov/multimedia/animations/
Kepler results: from powerpoint presentation by Bill Borucki http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=98
Kepler 34-b: from http://www.cfa.harvard.edu/news/2012-02
Kepler-453: from “Discovery of Tenth Tatooine-like Circumbinary Planet“ http://www.ifa.hawaii.edu/info/press-releases/Kepler453b/
Exoplanets bouncing in a 250AU binary: from http://www.nickolas1.com/
Habitable zone: from Wikipedia http://en.wikipedia.org/wiki/Exoplanets
Kepler’s small habitable zone planets: from http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=393
Kepler K2 mission: from http://keplerscience.arc.nasa.gov/K2/MissionConcept.shtml and http://kepler.nasa.gov/news/nasakeplernews/index.cfm?
FuseAction=ShowNews&NewsID=341
Planet in quadruple star system: from http://news.yale.edu/2012/10/15/armchair-astronomers-find-planet-four-star-system
SKA: from http://www.skatelescope.org/media-outreach/images/ and http://www.skatelescope.org/media-outreach/videos/
GMT: http://www.gmto.org/overview/
TMT: http://www.tmt.org/gallery/photo-illustrations
E-ELT: http://www.eso.org/public/images/archive/search/?adv=&subject_name=Extremely%20Large%20Telescope
Proton-proton chain: from Astronomy 162: Stars, Galaxies and Cosmology: The Proton-Proton Chain
http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html
Davis experiment: from “Big World of Small Neutrinos”, http://conferences.fnal.gov/lp2003/forthepublic/neutrinos/
Super-Kamiokande: Diagram: from Super-K at U. of Washington, http://www.phys.washington,edu/~superk/uwgroup.html
Photo of interior: from Photo Album of the SK detector, http://www-sk.icrr.u-tokyo.ac.jp/doc/sk/photo/index.html
Cherenkov radiation: from http://spectrum.ieee.org/energy/nuclear/nuclear-wasteland
Neutrino event from SN1987A: from John Vander Velde http://www-personal.umich.edu/~jcv/imb/imbp4.html#Supernova
Image distortion due to gravitational waves: from “Will we ever detect gravitational waves directly?” by Matthew Francis,
http://galileospendulum.org/2013/03/26/will-we-ever-detect-gravitational-waves-directly/
LIGO images: from http://www.ligo.caltech.edu/
VIRGO images: from http://www.ego-gw.it/public/about/whatIs.aspx
Westerlund 2: from “Hubble Space Telescope Celebrates 25 Years of Unveiling the Universe” http://hubblesite.org/newscenter/archive/releases/nebula/2015/12/
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