Class 19 : The EM spectrum and the discovery of compact objects

The electromagnetic (EM) spectrum. Electromagnetic waves. From radio waves to gamma rays.

Discovery of neutron stars. Discovery of black holes.

Have already come across relation of color to temperature (e.g., for star).

As temperature increases, go from

What is physical difference between red & blue light?

I : Electromagnetic spectrum

REDYELLOWWHITEBLUE

Light is waves of electric & magnetic field. Waves characterized by

Wavelength () = distance between crests. Frequency (f or ) = number of crests passing a given point

per second. Speed of a crest: c = f. Energy of light wave proportional to frequency.

Blue

Red

Visible light is small part of spectrum: = 300 nm for blue light. = 700 nm for red light.

But what about shorter and longer wavelengths?

Small wavelengthHigh frequency

High energy

Large wavelengthLow frequencyLow energy

Visible and infrared astronomy

Keck

Subaru

IRTF

CFHTGemini

JCMT

UKIRT

Radio/mm astronomy

Greenbank, WV

Ultraviolet & X-ray Astronomy

XMM

II : Discovery of Neutron Stars Jocelyn Bell & Anthony Hewish.

Radio astronomers at the University of Cambridge in England.

Interested in radio-emissions from “quasars.”

Bell found curious “scruff” in her signals… blips that were always 1.33728 sec apart.

What was the periodic signal? Rule out equipment fault, Earth orbiting

satellite, Little Green Men (LGM). Is, in fact, a spinning neutron star (pulsar).

From web site of Cambridge radio astronomy group

1 secSounds:

0.1 sec 1.5 ms

Pulsars… Now know of hundreds of pulsars. Fastest known have periods of 1.5-3 ms

(rotate 300-600 times per second!). Very active subject of research…

What is the structure of a neutron star? What determines how fast they spin? How do they beam emission? Magnetars.

Magnetars

Spinning neutron stars with incredibly strong magnetic fields.

Object Strength (Earth = 1)

Iron bar magnet 102

Sustained lab field 105

Strongest star 106

Strongest lab field 107

Typical pulsar 1012

Magnetar 1015

III : Discovery of black holes

1960s and 1970s… Early days of X-ray astronomy. Crude X-ray observatories placed on rockets (get few

minutes of data while rocket above atmosphere).

What did they expect? Knew that Sun was bright in X-rays. Expected X-rays from Moon (reflection).

What did they find? Bright unidentified sources (Sco-X1 and Cyg-X1).

Follow up study of Cyg-X1: Bright and variable X-ray emission. Found high-mass star at approximate

location of X-ray source. Found that star has very large velocity shifts

(40 m/s in each direction)… … must be something very massive

swinging it around!

Cyg-X1 was the first black hole candidate.

X-ray binaries

X-ray binaries: “Normal star” orbiting around a neutron-star

(NS) or black hole (BH). Gas gets pulled off normal star and spirals

onto the NS/BH. Gas heats up and emits X-rays as it falls

(like hydro-electric power).

Very powerful objects. Cyg-X1 is 105 times more powerful than the

Sun (and most energy emerges in X-ray band).

Insert cool movie here…

Zooming in on an X-ray binary

Reynolds & Armitage