CHAPTER 15: OUTER SPACE

Agcaoili, Mikaela F.

IV-31 BSE General Science

A. Measuring The Distance Of The Stars

B. Stellar Parallax Determinations

C. Spectroscopy And The Stuff Of The Stars

D. The Hertzprung-Russel Diagram

E. The Color Magnitude Relationship And Distance To

Stars

F. The Cepheid Distance Scale

A. Measuring The Distance Of The Stars

Astronomers used the basic

technique of TRIANGULATION

to measure the star distances.

By measuring the angular

distance between a star and the

distant background stars and

repeating the process 6months

later when the Earth is on the

opposite side of the sum. If you

know the length of the baseline

(the Earth's orbit) and the angle

then you know the distance to

the star.

Halley also made similar studies of this using the

planet Venus and improved the estimation of

Earth-Sun distance that we now know as

Astronomical Unit or AU (153 Million Km)

At the end of 18th Century, astronomers had a

good idea of the actual scale of the solar system

Parallax is a displacement or difference in the

apparent position of an object viewed along two

different lines of sight, and is measured by the angle

or semi-angle of inclination between those two lines.

B. Stellar Parallax Determinations

He made a series of smaller

holes in a screen facing the

sun, until he estimated the

light was of the same intensity

as that of the star Sirius.

(Incorrect Calculations)

He came with the calculated distance of Sirius to

an equivalent of 83,199 AU.

The Actual Distance of Sirius is 550,000

AU. They all got it wrong

It was only in 1830’s that measurement

of distances of the stars become more

accurate along with the empowering of

technology

1. Friedrich William Bessel (1838)

o used 61 Cygni

o Measured a distance of 0.3136 sec of arc

o Distance 10.3 light years away from us

Other studies using parallax method:

2. Scot Thomas Henderson

o Was in South Africa when he studies Alpha Centauri

o 1 parallax of 1 sec of arc

o Distance of 1.3 par sec; 4.3 light years away

Other studies using parallax method:

2. Scot Thomas Henderson

o Was in South Africa when he studies Alpha Centauri

o 1 parallax of 1 sec of arc

o Distance of 1.3 par sec; 4.3 light years away

2. Scot Thomas Henderson

o Was in South Africa when he studies Alpha Centauri

o 1 parallax of 1 sec of arc

o Distance of 1.3 par sec; 4.3 light years away

Other studies using parallax method:

3. Friedrich Von Struve

o Polka Observatory near St Petersburg (Russia)

o Vega (alpha Lyrae)

o Parallax: 0.2613 sec of arc

o Distance: 8.3 Parsec; 27 Light years away

The important thing to takeaway from all

these numbers is that all these stars are

considered our neighbours on a cosmic

level and yet they are so far away!

Old measurements were usually done using the ey

making it hard to have accurate measurement but at

the end of the 20th century the satellite HIPPARCOS

revealed parallaxes of 120,000 stars.

Hipparcos, a satellite with the

Hipparcos Catalogue and

other star datasets, was a

scientific mission of the

European Space Agency,

launched in 1989 and

operated until 1993.

Spectroscopy

C. Spectroscopy And The Stuff Of The Stars

The Hertzsprung–Russell diagram is a

scatter graph of stars showing the

relationship between the stars' absolute

magnitudes or luminosities versus their

spectral types or classifications and effective

temperatures.

D. The Hertzsprung-Russel Diagram

Hertzsprung–Russell diagrams are not maps of

the locations of the stars. Rather, they plot each

star on a graph measuring the star's absolute

magnitude (brightness) against its temperature

(color).

Hertzsprung–Russell diagrams are also referred

to by the abbreviation H–R diagram

The diagram was created circa 1910 by Ejnar

Hertzsprung and Henry Norris Russell and

represents a major step towards an

understanding the "the lives of stars".

Ejnar Hertzsprung

From Denmark

Trained in chemical engineering

and later studied photo chemistry

Work at the University of

Copenhagen to be an

observational astronomer and

applied photography to his

observations.

From New York

Studied at Princeton and

University of Cambridge

Had the same observation

as Hertzsprung and

published his work.

• Star Size: The bigger a star is the

hotter it will be.

• The stronger an object's gravity is,

the more power it has to pull its

mass inward. This causes the

core to be very compact and

creates a lot of extra

pressure. This extra pressure

builds up, raising the temperature

of the core. The hotter the core

gets, the more of its hydrogen fuel

it will burn.

• Star Life: The smaller a star is, the

longer it will live.

E. The Color Magnitude Relationship And Distance To Stars

The majority of stars on a typical HR diagram lie along the main-

sequence curve. This line is pronounced because both the spectral

type and the luminosity depend only on a star's mass, at least

to zeroth-order approximation, as long as it is fusing hydrogen at its

core—and that is what almost all stars spend most of their "active"

lives doing.

• After the WW1, Eddington

travelled to the island

of Príncipe near Africa to watch

the solar eclipse of 29 May

1919. During the eclipse, he

took pictures of the stars in the

region around the Sun.

• One of Erdington's

photographs of the total solar

eclipse of 29 May 1919,

presented in his 1920 paper

announcing its success,

confirming Einstein's theory

that light "bends"

The H-R diagram can also be used by scientists to roughly measure

how far away a star cluster is from Earth. This can be done by

comparing the apparent magnitudes of the stars in the cluster to the

absolute magnitudes of stars with known distances. The observed group

is then shifted in the vertical direction, until the two main sequences

overlap. The difference in magnitude that was bridged in order to match

the two groups is called the distance modulus and is a direct measure

for the distance. This technique is known as main-sequence

fitting or spectroscopic parallax.

F. The Cepheid Distance Scale

Hanrietta Swan Leavitt

Cepheid

• Hanrietta Leavitt compiled a list of 1,777

periodic variables. Eventually she classified

47 of these in the two clouds as Cepheid

variables and noticed that those with longer

periods were brighter than the shorter-period

ones.

• She plotted her observation and her plot

showed what is now known as the period-

luminosity relationship; cepheids with longer

periods are intrinsically more luminous than

those with shorter periods.

• Ejnar Hertzsprung

realized the importance

of her discovery. By

measuring the period of

a Cepheid from its light

curve, the distance to

that Cepheid could be

determined. He used his

data on nearby

Cepheids to calculate

the distance to the

Cepheids in the SMC as

37,000 light years away.

Some clusters of stars are grouped together with different masses,

colour and temperature, if there is one Cepheid in that cluster it is

possible to measure the distance of all the stars is known.

It was probing beyond the Milky way that Cepheid's altered our view of

our place in the Universe.