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Struktura a kinematika galaxií
Bruno Jungwiert
Přednášky č. 1 a 2 (22. 10. 2020)
1. Mléčná dráha – morfologie, složení, rozměry, hmotnosti
2. Gravitační pole galaxií, rotační křivka, dráhy hvězd
Přírodovědecká fakulta MU - F7567 - 2020
Hvězdná noc, 1889
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The Milky Way Galaxy
The Milky Way, our Galaxy,
as seen from the European Southern Observatory, Chile
http://www.eso.org/gallery/v/ESOPIA/Paranal/phot-33a-07.tif.html
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The Milky Way Sets
Image Credit & Copyright: Juan Carlos Casado (TWAN, Earth and Stars)
Explanation: Under dark skies the setting of the Milky Way can be a dramatic sight. Stretching nearly parallel
to the horizon, this rich, edge-on vista of our galaxy above the dusty Namibian desert stretches from bright,
southern Centaurus (left) to Cepheus in the north (right). From early August, the digitally stitched, panoramic
night skyscape captures the Milky Way's congeries of stars and rivers of cosmic dust, along with colors of
nebulae not readily seen with the eye. Mars, Saturn, and Antares, visible even in more luminous night skies,
form the the bright celestial triangle just touching the trees below the galaxy's central bulge. Of course, our
own galaxy is not the only galaxy in the scene. Two other major members of our local group, the Andromeda
Galaxy and the Triangulum Galaxy, lie near the right edge of the frame, beyond the arc of the setting Milky
Way.
credit: http://apod.nasa.gov/apod/ap160826.html
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European Southern Observatory
The Milky Way, our Galaxy (optical image), seen “edge-on”
(credit: ESO)
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The Milky Way Galaxy seen COBE/DIRBE JKL image
in the near infrared credit: NASA
The image shows a composite all-sky photo of the Milky Way Galaxy as taken by the NASA spacecraft COBE in 1993. The
photo clearly shows the disky nature of our Galaxy as well as the fact that we live close to the mid-plane of the disk. The
central puffy region is the Milky Way’s bar. Note that the dark patches – due to starlight absorption by interstellar dust –
appearing on optical images of the Milky Way (see previous slides) are considerably reduced (the IR radiation is absorbed by
dust much less than visible light).
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wikipedia.org
The
Milky
Way
The Milky Way’s
structure
as seen from “face”
in the starlight
(= visible light)
(artist's picture)
main structural
components seen
in this figure
(all made of stars):
- disk with
spiral arms
- bulge/bar region
The Sun is located
within one of fainter
spiral arms, so- called
Orion arm
(or Orion spur)
http://www.eso.org/
public/images/eso1339e339e/
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Bland-Hawthorn & Gerhard, 2016
The Milky Way’s bar
The Milky Way is a barred
spiral galaxy. The presence of its
bar can be deduced by counting
stars along different lines-of-sight
on its infrared image (previous
slide). For a bar tilted
with respect to the line joining
our position and the center of the
Galaxy, as is the case, the lines-ofsight
taken symmetrically on the left
and on the right side of the Galaxy
will result in different star counts.
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The Milky Way in a nutshell:
- a large barred spiral galaxy containing some 200 billion stars
- the total mass is estimated to be around 1.5 trillion (1.5 x 1012
) solar masses
- most of the mass, more than 90%, is believed to be in the form of dark matter
- only 3-4% of the mass are made of stars
- remaining 2-3% are gaseous
The Milky Way’s disk extends to roughly 100,000 l.y. (30 kpc) in radius.
Our Sun is at 27,000 l.y. (8.2 kpc) from the center of the Galaxy.
This is less than 1/3 of the disk radius, however more than 90% of all Milky Way’s stars are
located within the Sun’s nearly circular orbit.
The dark matter halo (see next slides) extends to 0.5 - 1 million light years (150 – 300 kpc).
94%
4%2%
Dark matter
Stars
Gas
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80%
19%
1%
Disk
Bar/bulge
Stellar halo
Stars are distributed in several structural components:
- stellar disk with spiral arms
- stellar bar (or bulge-bar)
- stellar halo (see next slide)
The total stellar mass is roughly
6 . 1010
solar masses.
Image credit: E. Hubble,
The Realm of the Nebulae
The Milky Way Galaxy is a barred
spiral galaxy, probably of the
Hubble type SBbc
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Image credit: By RJHall at English Wikipedia, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=52696960
The stellar halo and globular clusters
Apart from the disk and the central bar-bulge region,
the Milky Way (as well as other spiral galaxies) possesses
a large spherical stellar component, a stellar halo. Stars in
the halo appear either as dispersed individuals (so-called halo field
stars, or group in the so-called globular clusters.
tThe dark matter halo:
The stellar and gaseous disk of the Milky Way is believed to be embedded in a much larger dark matter halo having a diameter of
more than 1 million light years. The dark matter halo dominates the mass of the Galaxy.
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Image credit: Efremov 2011
Spiral galaxies thought to resemble the Milky Way Galaxy
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The Milky Way and
our Sun:
- the Sun is located 27,000
light years, or
1.7 billion AU, from the
Galactic center, close to the
mid-plane of the Milky Way
disk
- it orbits the Milky Way at a
speed of 240 km/s,
or 540,000 mph,
(8 times faster than the
orbital speed of the Earth
around the Sun (30 km/s))
- one orbit takes
220 million years
to complete (this is
sometimes referred to as the
Galactic year)
- since the age of the Sun is
4.6 billion years, the Sun
has accomplished just about
20 turns around the Galactic
center (assuming its orbit has
not changed too much)
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