Your Observing Challenge:White Dwarfs in Open Star Clusters

Star clusters are collections of stars born nearly-simultaneously. Once we know their ages, we can determine how different stars

change with time.

Messier 35 and NGC 2158 (CFHT)

47 Tucanae (HST)

Models tell us how the appearance of stars should change with time.

The ages of star clusters are usually determined by the “main sequence turn-off”

in a color-magnitude diagram.

The models that give white dwarf ages use different physics from the models that give us main sequence ages, giving us a sanity check!

von Hippel (2005)

Your Observing Challenge:

Take pictures of a star cluster with our 0.8-meter telescope and find candidate white dwarfs that might be useful for studying

ages of the cluster.

First, let’s examine your primary tools: the CCD camera and ImageJ

Light is the only probe we have for objects outside the

solar system.

Telescopes are funnels that collect large amounts of

light from stars, but must send light somewhere.

Several devices are capable of collecting light and

storing information about it.

The first modern astronomers used their eyes,

pencil and papers and brains.

William & Caroline Herschel

Heber CurtisMaria Mitchell

Eyes are very inefficient and cannot integrate.

Eyes can only do comparisons between

objects, and have trouble making quantitative

measurements.

The brain is not the most reliable image processing

software.

Around the turn of the century, photographic plates

became popular.

John Draper 1839Henry Draper 1880David Malin 1979

Plates can integrate and are modestly efficient.

Plates store images accurately for long periods

of time.

CCDs are the modern astronomer’s weapon of

choice for observing.

CCDs can integrate for long times at nearly 100%

efficiency.

The images are read into a computer and stored on

disk.

Dr. Jana Pittichova

CCDs pictures can be added together, letting you see

fainter.

Single Stack of 5

CCDs convert light to electrical signals.

CCDs are based on Einstein’s photoelectric effect.

CCDs are made of individual pixels, each of which works

independently.

The electrons knocked out of silicon are held in a well until

the exposure is finished.

QuickTime™ and aGIF decompressor

are needed to see this picture.

After the exposure, circuitry counts the number of

electrons and reports it to the computer.

If the signal is too high, the circuitry can’t count the electrons – the pixel is

“saturated.”

Colors are measured by taking images through colored glass filters.

CCDs only detect the number of photons, not their color.

Filters allow only one color of light through.

Measuring brightness through different filters

gives us color.

From Hubblesite.org

There are dozens of flavors of filters; the most common are the

Johnson system

U = ultraviolet

B = blue

V = green

R = orange/red

I = very red (not infrared) Wavelength

blue red

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CCDs are not perfect, so astronomers must take

calibration data every night.

These calibrations take care of most defects.

Images of “standard stars” allow images to be put on an

absolute scale.

Different telescopes and instruments have different

throughput.

Dust and haze change the amount of light reaching the

ground.

Calibration using standard stars lets us compare data from different telescopes

and nights.

With cameras using one or more CCDs, astronomers

have been study very large areas of sky.

Kitt Peak Mosaic

Keck Observatory LRIS

Steward Observatory 90Prime

Astronomy requires the collecting of light for later

study.

Astronomers need cameras that take long exposures and

are efficient.

CCDs fit the bill -- provided the right calibrations are

taken.