All figures in this lecture are from wikipedia.org pages
Struktura a kinematika galaxií (F7567)
Bruno Jungwiert
I. Stručná historie objevu galaxií
The Milky Way,
star clusters, nebulae and galaxies
The historical perspective:
1. From Galileo Galilei to Thomas Wright and Immanuel Kant
2. Catalogues of star clusters and nebulae:
From Messier’s and Herschel’s Catalogues to
the New General Catalogue
3. The Leviathan telescope and the first spiral nebulae
4. The advent of astrophotography:
From the Moon daguerrotype to the first photo of a spiral nebula
5. The Great Debate
Image credit: National Geographic, Babak Tafreshi
Galaxy, galaxies – Etymology:
γαλαξίας
κύκλος
(galaxias kyklos = milky circle)
Origine della Via Lattea (The Origin of the Milky Way), Tintoretto, c. 1575
1.
From Galileo Galilei to
Thomas Wright and Immanuel Kant
Galileo Galilei (1564 - 1642)
Galileo’s portrait by Tintoretto, 1607
In his book Sidereus nuncius (Starry messenger), published in 1610, Galileo Galilei
reports about his revolutionary discoveries (he made the first astronomical
observations of the sky using a telescope):
- craters and mountains on the Moon’s surface
- the phases of Venus
- four moons of Jupiter (later dubbed the Galilean moons)
- an extended structure around Saturn (later recognized to be a ring)
- dark spots on the Sun
- thousands of stars
- nebulae and star clusters (see next slides).
Watch the 1st
chapter of the movie Eyes on the Skies (400 years of telescopes):
https://www.youtube.com/watch?v=jRUaQFsDcJQ
He resolves the Milky Way light into individual stars unseen by the naked eye
and conjectures that the whole Milky Way band on the sky is made of faint and
distant stars. This idea is further developed by Thomas Wright, Immanuel Kant and
William Herschel (see next slides).
Some of the next slides show pages from the Galileo’s book (the Orion nebula, the
Beehive cluster, the Pleiades cluster): on them, symbols of stars visible by the naked
eye show a central dot, to distinguish them from many more stars seen by Galileo
through his telescope.
The Orion
constellation
and
Nebulosa
Orionis
(The Orion
Nebula, M42)
The Orion Nebula (M42)
(Galileo’s picture versus a modern photo
taken by the Hubble Space Telescope)
Image credit: NASA/HST
Notes:
- M42 is a diffuse emission nebula, located 1,350 light years away
- it is the nearest large region of ongoing star formation
- angular size: 1 degree (two full Moons), size of 25 light years
The star cluster Praesepe (Manger, or Beehive cluster, M44)
in the constellation of Cancer
Notes:
- Ptolemy (2nd
century A.D.): “nebulous mass in the breast of Cancer”
- Galileo (in 1610) sees at this place 40 stars with his telescope
- M44 is one of the nearest open clusters, located 600 light years away
- 700 million years old, it contains around 1,000 stars with a total mass equivalent
to approximately of 500 Suns
- angular size: 1.5 degrees (3 times the full Moon), size of 80 light years
Image credit: Sloan Digital Sky Survey
The Pleiades (The Seven Sisters, M45)
Pleiades by Elihu Vedder (1885)
Notes:
- M44 is one of the nearest open clusters, located 450 light years away
- 100 million years old, it contains over 1,000 stars
- angular size: 2 degrees (4 times the full Moon), size of 80 light years
El nacimiento de la Vía Láctea (The birth of the Milky Way), Peter Paul Rubens, c. 1637
Thomas Wright
(1711-1786)
1750:
An original theory or
new hypothesis of
the Universe
Thomas Wright’s
New hypothesis of the Universe
(1750):
The appearance of the Milky Way
explained as:
"an optical effect due to our
immersion in what locally
approximates to a flat layer of
stars."
Developing Wright’s ideas further:
this locally flat layer of stars could be
part of a larger, globally non-flat
system, possibly a spherical shell.
More spherical shells could be
nested inside each other.
Wright conjectures that many faint
nebulae are actually distant galaxies:
“… the many cloudy spots,
just perceivable by us, ...,
in which tho' visibly
luminous spaces,
no one star or particular
constituent body
can possibly be distinguished;
those in all likelihood may be external
creation,
bordering upon the known one,
too remote for even our telescopes to
reach.”
Immanuel Kant
(1724-1804)
1755:
Universal natural history and
theory of the heavens
Kant’s … Theory of the Heavens (1755):
Immanuel Kant speculates that the Milky Way might be a rotating body of a
huge number of stars, held together by gravitational forces akin to the Solar
System but on much larger scales (the Newton’s theory of gravity, successfully
explaining the planetary mechanics within our Solar System, emerges in
1680s).
The resulting disk of stars would be seen as a band on the sky from our
perspective inside the disk.
He also conjectures that some of the nebulae visible in the night sky might be
separate "galaxies" themselves, similar to our own. He refers to these
"extragalactic nebulae" as "island universes".
Image credit: Bruno Gilli/ESO - http://www.eso.org/public/images/milkyway/
2.
Catalogues of star clusters and nebulae:
From Messier’s and Herschel’s Catalogues
to the New General Catalogue
Charles Messier (1730-1817):
Catalog of nebulae and star clusters
(1st
version: 1774 – 45 objects,
final version: 1781 – 103 objects,
+ 7 objects added later)
Messier is a “comet hunter”. He creates his
catalog with the goal to make identification of
newly arriving comets easier.
Observations are made from Hotel du Cluny,
Paris.
Messier’s drawing ot the Orion nebula (M42)
Messier’s drawing of
the Andromeda nebula M31
(today known as
The Andromeda galaxy)
A modern photo of
the Andromeda galaxy
Image credit: By Adam Evans - M31,
the Andromeda Galaxy
https://commons.wikimedia.org/w/index.php?
curid=12654493
The Pleiades and
the comet Lovejoy C/2014 Q2
(Photo by Rick Bryant on 1/17/15)
The Pleiades and the Moon
http://www.derekscope.co.uk/moon-pleiades/
Messier 45 (M 45):
The Pleiades (Seven Sisters)
star cluster
The Nebra disk with Pleiades, 1600 B.C.
https://en.wikipedia.org/wiki/Nebra_sky_disk
A nebula, a galaxy or a star cluster ?
110 objects in the Messier’s Catalogue of nebulae and star clusters
(images in this mosaic contain modern photos of Messier’s objects)
Image credit: By Michael A. Phillips - http://astromaphilli14.blogspot.com.br/p/m.html official blog, CC BY 4.0,
https://commons.wikimedia.org/w/index.php?curid=38121043
Note:
The stars clusters, nebulae and galaxies contained in the Messier’s catalog are
often
referred to, even today, by their numbers in this catalog, preceded by capital M (for
Messier), for example:
M 1 = the Crab nebula
M 13 = the Great globular cluster in the constellation of Hercules
M 31 = the Great Andromeda nebula (the Andromeda galaxy)
M 33 = the Triangulum galaxy
M 42 = the Orion nebula
M 44 = the Praesepe (Beehive) cluster
M 45 = the Pleiades (Seven sisters) cluster
1 – Moon, 2 – Andromeda galaxy (M31), 3 – Triangulum galaxy (M33), 4 – Orion nebula (M 42), 5 – Lagoon nebula (M 5),
6 – Pinwheel galaxy (M 101), 7 – Sculptor (Silver coin) galaxy (NGC 253), 8 – Veil nebula (NGC 6960), 9 – SN1006,
10 – Helix nebula (NGC 7293), 11 – Sombrero galaxy (M 104), 12 – Crab nebula (M 1), 13 – Comet Hale-Bopp.
Color coding: galaxies – star forming nebulae – supernova remnants – planetary nebulae
Comparing angular sizes of the Moon and
nearby nebulae & galaxies
Image credit:
https://stargazerslounge.com/topic/208533-comparing-the-size-of-deep-sky-objects/
http://apod.nasa.gov/apod/ap160328.html
Star forming nebulae in the Orion constellation, seen above Pico del Teide volcano
Image credit: Stephen Rahn / Tom Buckley-Houston
The Moon (half a degree angular size) and the Andromeda galaxy (M31) (3 degrees).
The image is a collage of two separate photos: 1) an ordinary camera photo of the sky showing the Moon, Venus and Mercury;
2) a UV photo of the Andromeda galaxy taken by the NASA’s GALEX space telescope. Angular sizes are respected, positions are not.
William Herschel:
(1738 - 1822)
Catalogue of nebulae and clusters of stars
1786: 1,000 entries (Note:
1789: 2,000 entries in 1781, Herschel discovers Uranus,
1802: 2,500 entries the 7th
planet of our Solar system)
Image credit: By
http://wellcomeimages.org/indexplus/image/V0002731.htmlWellcome
Collection gallery (2018-04-03):
https://wellcomecollection.org/works/hs76suwh CC-BY-4.0, CC BY 2.0,
https://commons.wikimedia.org/w/index.php?curid=59410576
Herschel’s Great Fourty-Foot telescope, 1789
(48-in/120-cm mirror, 12-m focal length;
the largest mirror until 1845)
William Herschel's model of the Milky Way, 1784
(in: Account of Some Observations Tending to Investigate the Construction of the Heavens (Philosophical
Transactions of the Royal Society of London, Vol. 74, 1784)
William Herschel's model of the Milky Way, 1785
- the first attempt to describe the shape of the Milky Way
and the position of the Solar system in it; based on “star
gauging” (star counts) in more than 600 directions
(note: interstellar extinction due to dust is not known at that time; it will not be
discovered until 1930)
- W. H. also measures proper motions of 19 stars, finding
that the Sun is moving towards the constellation of
Hercules
John Herschel:
(son of William Herschel)
General catalogue of nebulae
and clusters of stars (GC)
1864: 5,079 objects
John Dryer:
New General catalogue of nebulae
and clusters of stars (NGC)
1888: 7,840 objects
Index Catalogues (IC)
1895 & 1908: additional 5,386 objects
3.
The Leviathan telescope and
the first spiral nebulae
Leviathan Telescope
(w/ 72-inch / 1.8-m mirror) – the largest telescope of its time
Parsonstown, Ireland (1845):
spiral nebulae discovered
by William Parsons (lord Rosse)
Image credit: Telescopes.stardate.org
To be compared to:
Galileo’s telescopes:
1.5-cm (1609); 2.6-cm (1612)
3.7-cm (1620)
William Herschel’s
40-foot telescope:
48-inch / 1.2-m (1789)
60-inch Hale telescope
60-inch / 1.5-m (1908)
Hooker telescope
100-inch / 2.5-m (1917)
Hale telescope
200-inch / 5-m (1949)
Hubble Space Telescope (HST)
94-inch / 2.4-m (1990)
Keck Telescopes
2 x 10-m (equivalent to a single 14m
telescope) (1993 + 1996)
Very Large Telescope (VLT)
4 x 8.2-m (equivalent to a single 16m
telescope) (1998)
James Webb Space Telescope
(JWST), 6.5-m (2022)
Giant Magellan Telescope
25-m (2029)
Extremely Large Telescope (ELT),
39-m (2027)
William Parsons’ (Lord Rosse’s)
drawings of spiral nebulae seen
through his Leviathan telescope:
M51 (1846)
M99 (1848)
But the true nature of spiral nebulae, today called
spiral galaxies (large systems composed of stars
and lying outside our own Galaxy, as independent
stellar islands), remains unknown until 1924.
http://messier.seds.org/more/m051_rosse.html
http://messier.seds.org/more/m099_rosse.html
left: Rosse's drawing
(1846) - of the first “spiral
nebula”, M51 (recognized as a
nebula already by Messier in his 1781
catalogue)
Right: a 20-th century
photo of M51 (now called
“Whirlpool galaxy”)
(see also its HST image –
next slide)
Vincent van Gogh, Starry Night, 1889
Colliding galaxies?
Just whirls in the wind? Or whirls in his mind?
Could Van Gogh have known about galaxies by 1889?
Starry Night Scavenger Hunt Image Credit: Vincent van Gogh;
Digital Collage & Copyright: Ronnie Warner
Camille Flammarion:
Popular Astronomy,
1880
That’s where van
Gogh could have
seen pictures of
spiral nebulae.
4.
The advent of astrophotography
Louis Daguerre, 1838:
The first photography of human beings (Paris, Boulevard du Temple).
Exposure time: 5 minutes
John Draper, 1840:
The first astrophotography –
a dageurreotype of the
Moon.
Henry Draper, 1880: The first photo of a nebula ever taken – the Orion nebula.
A 51-minutes exposure with 11-inch (28 cm) Alvan Clark’s reflecting telescope.
Andrew Common, 1883: a photograph of the Orion nebula, the first to show that a
long exposure could record stars and nebulae invisible to the human eye (a 60-minute
exposure with a 36-inch (91 cm) telescope).
Isaac Roberts, 1888:
The first photography of a “spiral nebula”.
The Great Andromeda nebula
(today known as The Andromeda galaxy).
Exposure taken with a 20-inch reflecting
telescope.
5.
From nebulae to galaxies:
“The Great Debate”
National Academy of Sciences, Washington, April 26th
, 1920:
A debate on the Scale of the Universe
between Harlow Shapley and Heber Curtis
(it later became known as “The Great Debate”)
“Large galaxy” hypothesis vs “Island universes” hypothesis
“The Great Debate” of 1920
(see https://en.wikipedia.org/wiki/Great_Debate_(astronomy))
Harlow Shapley (1885 – 1972) Heber Curtis (1872 – 1942)
Models of our Galaxy in 1900-1920:
- diameter between 7,000 – 30,000 l.y.
- flattened (disk shaped)
- the Sun near its center
- possibly with spiral structure
Image credit: Trimble, V., 1995Image credit: Trimble, V., 1995
By 1920, our Galaxy represents the whole
known Universe. It is not clear whether
anything lies beyond it.
In general, astronomers at that time agree
that the Galaxy is a flattened disk of stars
with the Sun at or near its center (“S” in the
left “face-on” view of the disk, the cross in
the above “edge-on” view). The diameter of
the disk is estimated, at that time, to be
30,000 light years at most (for comparison,
the distance to the nearest star is 4 l.y.; the
diameter of the outermost planet in the
Solar system, Neptune, is 0.001 l.y).
Shapley suggests that our Galaxy is much bigger than
thought, with a diameter of 300,000 light years (the
modern value for the diameter of the stellar disk of our
Galaxy is close to 200,000 l.y.).
He also places the Sun and its planetary system far
(65,000 l.y.) from the Galaxy’s center
(the modern value is 27,000 l.y.)
He prefers to think that spiral nebulae are gaseous objects
inside our Galaxy.
Curtis believes that the Galaxy is much smaller, with a
diameter not exceeding 30,000 l.y. He keeps the Sun close
to the center. In his opinion, spiral nebulae are
extragalactic island universes made of stars – galaxies
like our own.
Shapley bases his suggestions on the observed
distribution of globular clusters for which he measures
distances using the period-luminosity relation of Cepheid
stars (the same relation Edwin Hubble used a few years
later to determine the distance of the Great Andromeda
nebula/galaxy). He notices that the center of the globular
cluster system is shifted with respect to what is considered
to be the center of our Galaxy (top image). He believes that
the two centers in reality coincide (bottom) which makes its
Galaxy larger and at the same time drives the Sun well off
its center.
5.b
Galaxies - island universes
The Andromeda nebula, photographed at the Yerkes Observatory around 1900.
Is it a gaseous nebula inside our Galaxy or an independent galaxy (a stellar
“island” external to our Galaxy)? Until 1924, nobody knows.
Image credit: Carnegie Observatories /
Huntington Library
In October 1923 (a year before the announcement is published in the NYT), Edwin
Hubble finds a Cepheid variable star in the Andromeda nebula.
Using the Period-Luminosity relation for Cepheids
(https://en.wikipedia.org/wiki/Period-luminosity_relation), he estimates the distance of the
nebula to be 930,000 light years (37% of the modern value of 2.5 million l.y.), well beyond
our Galaxy periphery. The linear size of the known Universe thus grows by a factor of ~ 30
(and the corresponding volume by a factor of 30x30x30, or nearly 30,000). At the same
time, it appears clear that spiral nebulae (and perhaps some non-spiral ones) are indeed
external stellar islands as hypothesized by Kant in 1755, some 170 years earlier.
For the story of the Cepheid variables and their famous Period-Luminosity relation, watch:
https://www.youtube.com/watch?v=QcChCeX2VrY
https://www.youtube.com/watch?v=E9gvk_OkrPw
Summary:
the timeline of main hypotheses and discoveries related to galaxies
1609 – Galileo Galilei builds a 1.5-cm aperture telescope and points it to the skies. Among others, he resolves some
parts of the Milky Way into individual stars. In 1610, he publishes Sidereus Nuncius (Starry Messenger) in which he
suggests that all the Milky Way light originates in myriads of distant stars, too faint to be seen individually.
1750 – Thomas Wright explains, in his Original theory or new hypothesis of the Universe, the Milky Way’s appearance
as “an optical effect due to our immersion to what locally approximates to a flat layer of stars. He also speculates that
“many cloudy spots (= nebulae) may be external creations too remote for even our telescopes to reach”.
1755 – Immanuel Kant, in Universal natural history and theory of the heavens, speculates that the Milky Way might be
a rotating body of a huge number of stars, held together by gravitational forces in a similar way as the Sun and its
planets in the Solar System but on much larger scales. The resulting disk of stars would be seen as a band on the sky
from our perspective inside the disk. Kant also conjectures that some of the nebulae visible in the night sky might be
separate "galaxies" themselves, similar to our own. He refers to these "extragalactic nebulae" as "island universes".
1785 – William Herschel presents the first scientific model of the Milky Way based on star counts in hundreds of
directions. The model is a flat disk with an irregular boundary; the Sun is located off-center.
1845 – William Parsons (lord Rosse) discovers several “spiral nebulae” with his 72-inch (1.8-m) Leviathan telescope,
the largest astronomical instrument built until then. The nature (composition) and distances of those nebulae remain
unknown.
1888 – Isaac Roberts takes the first photo of a spiral nebula (The Great Andromeda Nebula).
1912 – Vesto Slipher measures, for the first time, redshifts of a spiral nebula. Redshift can be converted to speed via
so-called Doppler formula.
1920 – “The Great Debate” is held between Harlow Shapley and Heber Curtis about the size of the Milky Way Galaxy
(at that time equivalent to the whole known Universe) as well as about the nature of spiral nebulae (gaseous or
starry?, internal or external to our Galaxy?).
1924 – Edwin Hubble settles The Great Debate issues by announcing the discovery of an inividual star within the
Great Andromeda nebula and the measurement of its distance (930,000 l.y., later corrected to 2.5 million l.y.). Soon,
other individual bright stars are resolved, and their distances measured, in the Great Andromeda nebulae and a few
other spiral nebulae. Spiral nebulae (henceforth called “spiral galaxies”) are identified as stellar islands external to our
Galaxy, confirming both the 1920 Curtis’ view and the 1755 Kant’s conjecture.
1926 – Edwin Hubble presents the first morphological classification of galaxies (later known as the Hubble sequence
or the Hubble tuning fork), dividing them into elliptical galaxies, lenticular galaxies and spiral galaxies (this latter class
being in turn subdivided into normal spirals and barred spirals).
1927 – George Lemaître for the first time interprets measurements of galactic redshifts (speeds) and distances as a
sign of an expanding Universe. In doing so, he relies on the General Theory of Relativity, a new theory of gravity,
formulated by Albert Einstein in 1915.
1929 – Edwin Hubble publishes, independently of Lemaître’s work, his own discovery of the expanding Universe. The
relation between distance and speed of galaxies becomes known as Hubble’s law (or Hubble’s relation). The idea of
an expanding Universe naturally leads to a Universe with a beginning (now referred to as the Big Bang). Between 1929
to 1998 it was believed that the Universe had been expanding – since the Big Bang – in a decelerated fashion, the
expansion being slowed down by gravity of all the matter found within galaxies (initially, from 1924, galaxies were
believed to be essentially stellar systems; later on, starting from 1951, various forms of gas were discovered in them,
in addition to stars; since 1970s, existence of dark matter, an invisible component dominating masses of galaxies,
was established).
1998 – The expansion of the Universe is found to be accelerating rather than decelerating This is being explained by
the existence of dark energy, another invisible component of the Universe. It appears that 68% of the Universe’s
content is made of dark energy, 27% of dark matter and only 5% by baryonic matter.