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 REDYELLOWWHITEBLUE

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 wavelength High frequency High energy Large wavelength Low frequency Low energy

Visible and infrared astronomy Keck Subaru IRTF CFHT Gemini 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 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 sec Sounds: 0.1 sec1.5 ms

Pulsars… Now know of hundreds of pulsars. Fastest known have periods of ms (rotate 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. ObjectStrength (Earth = 1) Iron bar magnet10 2 Sustained lab field10 5 Strongest star10 6 Strongest lab field10 7 Typical pulsar10 12 Magnetar10 15

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 10 5 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