Presentation ASTRONOMY1

One second of arc (arcsec) is an angle of only 1/3600 of a degree

SLIDE 2 This illustration shows the three steps astronomers used to
measure the universe's expansion rate to an unprecedented accuracy,
reducing the total uncertainty to 2.4 percent.

Astronomers made the measurements by streamlining and strengthening

the construction of the cosmic distance ladder, which is used to measure
accurate distances to galaxies near and far from Earth.

Beginning at left, astronomers use Hubble to measure the distances to a

class of pulsating stars called Cepheid Variables, employing a basic tool of
geometry called parallax. This is the same technique that surveyors use to
measure distances on Earth. Once astronomers calibrate the Cepheids'
true brightness, they can use them as cosmic yardsticks to measure
distances to galaxies much farther away than they can with the parallax
technique. The rate at which Cepheids pulsate provides an additional fine-
tuning to the true brightness, with slower pulses for brighter Cepheids. The
astronomers compare the calibrated true brightness values with the stars'
apparent brightness, as seen from Earth, to determine accurate distances.

Once the Cepheids are calibrated, astronomers move beyond our Milky
Way to nearby galaxies [shown at center]. They look for galaxies that
contain Cepheid stars and another reliable yardstick, Type Ia supernovae,
exploding stars that flare with the same amount of brightness. The
astronomers use the Cepheids to measure the true brightness of the
supernovae in each host galaxy. From these measurements, the
astronomers determine the galaxies’ distances.

They then look for supernovae in galaxies located even farther away from
Earth. Unlike Cepheids, Type Ia supernovae are brilliant enough to be seen
from relatively longer distances. The astronomers compare the true and
apparent brightness of distant supernovae to measure out to the distance
where the expansion of the universe can be seen [shown at right]. They
compare those distance measurements with how the light from the
supernovae is stretched to longer wavelengths by the expansion of space.
They use these two values to calculate how fast the universe expands with
time, called the Hubble constant.
Credit: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)

Normally could used handy guide to measuring the distance, 3

method for today…
SLIDE 3 Here’s our outlines for hole presentation FIRST – SE…

SLIDE 4 The first stellar parallax measurement was made in 1838 by Friedrich
Wilhelm Bessel, a German astronomer and mathematician. He found the parallax
angle of the star 61 Cygni to be 1⁄3 arcsec, and so its distance is about 3 pc

SLIDE5 * Consider what happens when two people look at a

nearby object from different places—they see it in different
positions relative to the things behind it. Furthermore, their
heads face at different angles when looking at it. This variation
in angle that occurs when viewing a nearby object from
different locations is called parallax (Figure 2-6).
 Using Parallax to Determine Distance (a, b) Our eyes change
the angle between their lines of sight as we look at things
that are different distances away. Our eyes are adjusting for
the parallax of the things we see. This change helps our
brains determine the distances to objects and is analogous
to how astronomers determine the distance to objects in
space
Hold a pencil vertically in front of and close to your nose. Focus
on distant objects or a background wall. Close one eye and
then open it while closing the other. Notice how much your
pencil appears to shift with respect to the distant objects. This
is called parallax: the apparent shift of a foreground object
with respect to background objects due only to a change in
the observer's position.
SLIDE 5-6: The first successful detections of stellar parallax were in the
year 1838, when Friedrich Bessel in Germany (Figure 19.5), Thomas
Henderson, a Scottish astronomer working at the Cape of Good Hope,
and Friedrich Struve in Russia independently measured the parallaxes
of the stars 61 Cygni, Alpha Centauri, and Vega, respectively -> LET SEE
THE MECHANISM OF THE CONCEPTP OF PARALLAX
SLIDE 6 Figure 19.6 Parallax. As Earth revolves around the Sun, the
direction in which we see a nearby star varies with respect to distant
stars. We define the parallax of the nearby star to be one half of the
total change in direction, and we usually measure it in arc seconds
SILDE 7: As Earth orbits the Sun, a nearby star appears to shift its position
against the background of distant stars. The star’s parallax angle ( p ) is equal to
the angle between the Sun and Earth, as seen from the star. The stars on the
scale of this drawing are shown much closer than they are in reality. If drawn to
the correct scale, the closest star, other than the Sun, would be about 5 km (3.2
mi) away. (d) The closer the star is to us, the greater the parallax angle p . The
distance to the star (in parsecs) is found by taking the in.verse of the parallax
angle p (in arcseconds), d 1/ p . (a, b: Mark Andersen/ JupiterImages)
To convert between the two distance unit s, just bear in mind: 1 parsec = 3.26
light-year, and 1 light-year = 0.31 parsec.

*where d is the distance to the star and p is the parallax angle of that
star. The equation is only this simple in these units, which is one of
the main reasons why many astronomers discuss cosmic distances in
parsecs rather than light-years. We will continue to primarily use
light-years (ly) throughout this book, however, as they are more
intuitive. In lightyears, this same equation becomes approximately
SLIDE 9 One of the most difficult things about precisely measuring the tiny angles
of parallax shifts from Earth is that you have to observe the stars through our
planet’s atmosphere. The effect of the atmosphere is to spread out the points of
starlight into fuzzy disks, making exact measurements of their positions more
difficult. Astronomers had long dreamed of being able to measure parallaxes
from space, and two orbiting observatories have now turned this dream into
reality.
Parallax measurements made in space thus enable astronomers to determine the
distances to stars well beyond the reach of ground-based observations.

SLIDE 10 Figure 19.8 H–R Diagram of Stars Measured by Gaia

and Hipparcos. This plot includes 16,631 stars for which the
parallaxes have an accuracy of 10% or better. The colors
indicate the numbers of stars at each point of the diagram,
with red corresponding to the largest number and blue to the
lowest. Luminosity is plotted along the vertical axis, with
luminosity increasing upward. An infrared color is plotted as a
proxy for temperature, with temperature decreasing to the
right. Most of the data points are distributed along the
diagonal running from the top left corner (high luminosity,
high temperature) to the bottom right (low temperature, low
luminosity). These are main sequence stars. The large clump
of data points above the main sequence on the right side of
the diagram is composed of red giant stars. (credit:
modification of work by the European Space Agency)
SLIDE 10-11 For a long time, the measurement of parallaxes and accurate stellar
positions was a backwater of astronomical research—mainly because the
accuracy of measurements did not improve much for about 100 years. However,
the ability to make measurements from space has revolutionized this field of
astronomy and will continue to provide a critical link in our chain of cosmic
distances.
Despite the information gained from stellar parallax, astronomers need to know
the distances to more remote stars for which parallax cannot yet be measured.
SLIDE 13 RS Puppis is one of the brightest known Cepheid variable stars in the
Milky Way galaxy

Two Supernovae in NGC 664 In 1997 the rare occurrence of two

supernovae in the same galaxy at the same time was observed in the
spiral galaxy NGC 664, located about 300 Mly (90 Mpc) from Earth.
Supernovae observed in remote galaxies are important standard
candles used by astronomers to determine the distances to these
faraway objects. The two supernovae overlap each other, as shown.
The upper, yellow-orange supernova was observed to occur 2 months
before the hotter, blue one, which was observed to occur less than 2
weeks before this image was made and had not yet achieved maximum
brightness. (Perry Berlind and Peter Garnavich, Harvard Smithsonian
Center for Astrophysics)
SLIDE 14 Cepheids are important to astronomers because there is a direct
relationship between a star’s period of pulsation and its average luminosity. This
relationship is called, appropriately enough, the period-luminosity relation.
SLIDE 15 First, we observe its period. Then we use the period luminosity relation
to determine its luminosity and, hence, its absolute magnitude (recall that the
luminosity and absolute magnitudes are directly related to each other). Then we
observe its apparent magnitude. Finally, we use the distance-magnitude
relationship in An Astronomer’s Toolbox 11-3 to calculate its distance.
SLIDE 15,16,17 Dim Cepheid variables pulsate rapidly, with periods of 1 to 2 days,
and have average brightness variation of a few hundred times our Sun’s
luminosity. The most luminous Cepheids have the longest periods of all Cepheids,
with variations occurring over 100 days and average brightness variations
equaling 10,000 L. Being so bright, we can see Cepheids far beyond the
boundaries of our Milky Way Galaxy. Because the changes in brightness of
Cepheids can be seen at distances where other techniques for measuring
distance, such as stellar parallax, fail, the period-luminosity relation plays an
important role in determining the overall size and structure of the universe. We
will explore this application further in Chapter 16.
The details of a Cepheid’s pulsation depend on the abundance of heavy elements
in its atmosphere. The average luminosity of metal-rich Cepheids is roughly 4
times greater than the average luminosity of metal poor Cepheids that have the
same period. Thus, there are two classes: Type I Cepheids (also called
Cepheid stars), which are the brighter, metal-rich stars, and Type II Cepheids (also
called W Virginis stars), which are the dimmer, metal-poor stars. The period-
luminosity relations for both types of variables are shown in Figure 12-27
The Period-Luminosity Relation The period of a Cepheid variable is directly related
to its average luminosity: The more luminous the Cepheid, the longer its period
and the slower its pulsations. Type I Cepheids (
Cepheid stars) are metal-rich stars. They are brighter than the Type II Cepheids (W
Virginis stars), which are metal-poor stars. (Adapted from H. C. Arp)
SLIDE 19 This illustration shows the rhythmic rise and fall of starlight from the
Cepheid variable star V1 over a seven-month period.
Cepheid variables are pulsating stars that brighten and fade in a predictable
pattern. The illustrated graph shows that V1 completes a pulsation cycle every
31.4 days. The red dots on the graph represent observations by the American
Association of Variable Star Observers (AAVSO), who partnered with the Space
Telescope Science Institute's Hubble Heritage Project to study the star. The four
stars on the graph denote observations made by the Hubble Space Telescope's
Wide Field Camera 3.
Ten amateur astronomers from around the world made 214 observations of V1
between July 2010 and December 2010, obtaining four pulsation cycles. The
AAVSO study allowed the Hubble Heritage team to target Hubble observations
that would capture the star at its brightest and dimmest phases.
Credit:
Illustration Credit: NASA, ESA and Z. Levay (STScI). Science Credit: NASA, ESA,
the Hubble Heritage Team (STScI/AURA) and the American Association of
Variable Star Observers

SLIDE21 FIGURE 15-4 The Period-Luminosity Relation This graph shows the
relationship between the periods and average luminosities of classical (Type I)
Cepheid variables and the closely related RR Lyrae stars (discussed in Chapter 12).
Each dot represents a Cepheid or RR Lyrae whose luminosity and period have
been measured.
SLIDE 25 Figure 19.15 Luminosity Classes. Stars of the same temperature (or
spectral class) can fall into different luminosity classes on the Hertzsprung-Russell
diagram. By studying details of the spectrum for each star, astronomers can
determine which luminosity class they fall in (whether they are main-sequence
stars, giant stars, or supergiant stars).

Themostwidelyusedsystemofstarclassificationdividesstarsofagivenspectralclassint
osixcategoriescalled luminosity classes. These luminosity classes are denoted by
Roman numbers as follows: • Ia: Brightest supergiants • Ib: Less luminous
supergiants • II: Bright giants • III: Giants • IV: Subgiants (intermediate between
giants and main-sequence stars) • V: Main-sequence stars The full spectral
specification of a star includes its luminosity class. For example, a main-sequence
star with spectral class F3 is written as F3 V. The specification for an M2 giant is
M2 III. Figure 19.15 illustrates the approximate position of stars of various
luminosity classes on the H–R diagram. The dashed portions of the lines represent
regions with very few or no stars.
SLIDE 26 Together ,these make up the arsenal of information we need to trace
the evolution of stars from birth to death, the subject.
SLIDE 28: APPLICATION

Armed with this knowledge, the team estimated the

distance of 2,431 Cepheid variables in the Milky Way and,
using their position in the sky, were able to create an
important 3D map of our galaxy. This gave them a good
idea of what our galaxy actually looks like. But a new,
detailed 3-D map of the galaxy puts a twist in that image,
literally. It turns out that the galaxy is not a flat pancake
but warped with the edges curling above and below the
galactic plane
Read more: https://www.smithsonianmag.com/smart-news/new-3d-map-shows-milky-
ways-big-twist-180972797/#fHKUREHSZUQxslYD.99