13) Exploring Starlight ✅ Flashcards
What is magnitude?
Magnitude is the measure of a brightness of a star
What is the scale of magnitude?
The magnitude difference of 1 corresponds to the fifth root of 100≈2.5
What is apparent magnitude?
Apparent magnitude is how bright a star seems from Earth
What information can be obtained from a stellar spectrum?
- Chemical composition
- Temperature
- Radial velocity
What factors affect apparent magnitude?
- The total energy by the star in the visible region
- The distance to the star
- The amount of interstellar gas and dust that reflects and absorbs light
- The amount of light absorbed and scattered by the Earth’s atmosphere
What is absolute magnitude?
Absolute magnitude is how bright a star would be if all stars are 10 parsecs away
How do you calculate absolute magnitude?
M=m+5- 5log(d)
M=absolute magnitude
m=apparent magnitude
How do astronomers find the chemical makeup of a star?
Spectroscopy
What is spectroscopy?
Spectroscopy is when you collect light with the aid of a telescope and then diffract it to create a spectrum.
How do you know what chemicals are in a star?
When you do a spectroscope, there are a set of darker lines. The spectral lines correspond to exact wavelengths at which atoms in the outer layers of stars absorb light; each element has its own unique set of wavelengths
What do astronomers classify stars in?
Spectral types
How do astronomers classify spectral types?
Astronomers examine the ratios of
hydrogen: helium: other elements
What are the different Spectral Types?
O B A F G K M
What is the H-R Diagram?
The H-R diagram is a scatter graph the classify’s stars in accordance to their luminosity, spectral type, colour, temperature and evolutionary stage
What is the axis in the HR labelled as?
- Luminosity
- Temperature (in K)
- Absolute magnitude
- Spectral types
Where will the following go on the HR diagram:
- Main sequence stars
- The sun
- Red and blue giant stars
- White dwarf stars
- Supergiant stats
And what is the axis labelled as + where does it go?
What is plotted against each other on a H-R Diagram?
A star’s luminosity or absolute magnitude is plotted against either spectral type or temperature
Where are the main sequence stars found on the H-R Diagram?
The main sequence stars lie in a band running from top left to bottom right
Where are Giants and Supergiants found on the H-R Diagram?
Giants and Supergiants stars lie above the main sequence band
Giant are what colour?
Red or Blue
Where are White dwarf stars found on the H-R diagram?
White dwarfs lie below the main sequence band towards the left
How long is one light year?
1 l.y = 9.5 × 10 ¹²
How long is one parsec?
1 pc = 3.1 × 10 ¹³
What is a parsec?
It is the distance that a star would have to move that would cover an arc second in the sky as viewed from Earth
How long is one parsec in accordance to light years?
1 pc = 3.26 l.y
What are minutes of arc?
Minutes of Arc is one degree split into 60 minutes of arc. 1 arc minute is further split into arc seconds where one degree is 3600 arc seconds
What technique can we use to figure out the distance to a near star?
Heliocentric Parallax
What is Parallax?
Parallax is when the position of a near object differs greater in comparison to distant objects when viewed from different places
How do you calculate the size of a star?
Using Stefans law
What are variable stars?
Variable stars are stars that vary in brightness
Why do stars appear to vary in brightness?
- Changing physical quantity such as size
- Changes in the light reaching Earth
What are light curves?
Graphs of apparent magnitude against time
What are the two types of variable stars?
Intrinsic and Extrinsic variables
How does a short/long period light curve look like?
What does an eclipsing binary light curve look like?
What does a Cepheid variable light curve look like?
What does a novae & supernovae variable light curve look like?
What type of stars are usually short-period variable stars?
Eclipsing binaries, cepheids or pulsating stars that changes magnitude over several days or weeks.
What type of stars are usually long-period variable stars?
Giant stars with periods between a hundred and a thousand days.iant stars with periods between a hundred and a thousand days.
What relationship do Cephid variable stars have + what does this mean?
Cepheids have a period-luminosity relation. The longer the period between peaks of brightness the brighter the star is.
What are Cephied Variables?
Cephied Variables are when the Star actually pulsates. When it’s larger, it has a greater magnitude, and therefore it’s brighter
What are binary stars?
Binary stars are when there is a bright primary star and a dimmer secondary star that orbit around their mutual centre of gravity
What happens when the secondary star eclipses (moves in front of) the primary star?
When the secondary eclipses the primary, there is a large drop in intensity
What happens when the primary star eclipses the secondary star?
When the primary star eclipses the secondary star, there is a small drop in intensity
How would you find the period of an eclipsing binary star?
By charting the brightness of a binary star you can find its period. In the chart on the right, the period would be from the ‘dips’ at 24 and 64 leading to a period of 30 hours.
Why does a Nova occur?
In a binary system - one star is a white dwarf & the other is a large yellow or red giant, the dwarf has stronger gravity and pulls matter from the giant.
Eventually, the dwarf can’t hold onto the excess solar material & explodes this matter off its surface.
—> Build-up takes place over a short period of time.
Explain the Nova light curve.
When the star explodes, the luminosity increases by many magnitudes. The star reverts back to its former brightness after a period of 30 to 100 days.
What is the brightest event in the universe?
A supernova
What are the 2 types of supernovas?
1) SIMILAR TO NOVA - a dwarf takes material from a giant. This time the explosion destroys the dwarf. Typically, this takes place when the mass of the white dwarf is over 1.4 solar masses.
2 GREATER MASS THAN 8 SOLAR MASSES - The red giant swells so much it collapses in on itself. These are dramatic events as once they explode the core forms a neutron star or a black hole.
Explain the Supernova light curve.
The light curve shows a drastic increase in brightness before receding to a small luminance after a few months.
How do radio telescopes work?
Radio telescopes consist of a large concave dish that reflects and focuses radio waves into an antenna. Here, the antenna converts the radio waves into electrical signals that can be stored and processed
Why are radio telescopes built with large apertures?
Radio telescopes are large so they can achieve the best possible resolution by collecting as much radio waves as possible
What is Aperture Synthesis?
Aperture synthesis is when multiple telescopes are linked electronically to study the same source of radio waves.
This converts many telescopes into one large one with an aperture equivalent to the largest distance between individual telescopes
Why are most radio telescopes located on high mountains?
The air is dry and steady. Also, there is a lack of clouds which obscure your view
Where is the optimum place to put a telescope?
Space
Why is space the best place to put a telescope?
The satellite is above atmosphere where there is:
- no air to blur or absorb light
- no day-night cycle where your viewing times are restricted
- no light pollution from skyglow
meaning it is in the optimum position to place a telescope in space
What 2 wavelengths of electromagnetic radiation does Earth’s atmosphere NOT block?
Visible light and radio waves
What are the disadvantages of having a satellite telescope?
Space telescopes have limited lifetimes, very difficult to construct, launch and maintain.
Very expensive
What are the advantages of having a satellite telescope?
Clearer observations, No limitations to observing at nighttime, wider wavelengths of the electromagnetic spectrum can be observed
What can an orbital observing platform include?
Includes:
- Satellites
- High-altitude observing systems such as the aircraft of the Kuiper Airbourne Observatory
- HIREGS
What discoveries were first made using radio waves?
- Quasars
- The structure and rotation of our galaxy
- Pulsars
- Protoplanetary discs
- Jets from black holes
- SETI (Search for extraterrestrial intelligence)
What discoveries were first made using Infrared (IR)?
- Protostars
- Interstellar dust and molecular clouds
- ‘hotspots’ on the moon
What discoveries were first made using ultraviolet (UV)?
Corona and chromosphere structure of young stars
What discoveries were first made using X-ray?
- Active galaxies
- accretion disks surrounding black holes
- supernova remnants
What discoveries were first made using gamma(γ) rays?
Gamma ray bursts in distance galaxies
Where must other wavelengths observatories be put?
All wavelengths other than optical or radio, need to be put on fixed observing platforms orbiting the Earth. However, Infrared observatories can be sited on high mountains.
What is the distance modulus formula?
M = m + 5 - 5log(d)
M=absolute magnitude
m=apparent magnitude
d= distance
How can you use Spectroscopy and the H-R diagram to determine distances to stars?
First, you obtain a spectrum of the star and determine its spectral class. Then, using the H-R diagram you get its absolute magnitude (you use the main sequence band). You then observe the star to get its apparent magnitude. Finally, you use the distance modulus formula to calculate 𝗱
What are the first 5 letters in the greek alphabet?
α (alpha), β (beta), γ (gamma), δ (delta), ε (epsilon)
What is light intensity proportional to?
light intensity is proportional to 1
. . . . . . . . . . . . . . . . . . . . . . . .————–
. . . . . . . . . . . . . . . . . . . . . . . .distance²
How do calculate the ratio of light intensity of two stars?
intensity of light from α . (distance β)²
——————————— = ——————
intensity of light from β . (distance α)²
What is plotted against what in a typical Hertzsprung-Russell diagram?
Luminosity against Spectral type
How does temperature and absolute magnitude increase in a H-R diagram?
Temperature increases to the left and Absolute magnitude increases downwards
What spectral type is the Sun?
G2
How are absorption lines connected to the chemical composition of a star?
Light passing through the outer layers a star is made of all wavelengths / energy. Some of this light has the correct wavelength/energy to excite atoms in the star’s outer layers.
When the atoms de-excite they emit radiation/photons of the same wavelength/energy but in a random direction and not necessarily ‘outwards’. The wavelengths / energies at which these ‘reactions’ occur depends on the chemical element. Light received on Earth is dimmer and so darker at these wavelengths
What is the intensity of light proportional to?
d²
What type of variable star is used by astronomers to determine distances to stars?
Cepheid variable
How do we use cephid variable stars to figure out distances between us and the galaxy?
Figuring out the apparent brightness (and from that how far away it is)
Cepheids in other galaxies enable us to accurately measure how far away that galaxy is
Why can’t we use heliocentric parallax to figure out distances to far galaxies?
And what technique do we use instead?
Because the angles between us and them are extremely small
We use Cehphid variable stars to figure our distance instead
What is a Cataclysmic variable star?
Cataclysmic variable stars are stars which irregularly increase in brightness by a large factor, then drop back down to a inactive state.
What is an example of Cataclysmic variable star?
Supernova
Why is there variability in brightness for eclipsing binary stars?
For most of the time, the primary and secondary stars are not aligned. This is their constant brightness.
The larger/primary ‘dip’ in a light curve occurs when the
dimmer/secondary star is directly in front of the brighter/primary star.
The smaller/secondary ‘dip’ occurs when the secondary star is behind the primary star.
This time, the corresponding drop in intensity is not as significant
What is SKA?
Square Kilometre Array
What is aperture synthesis?
The process of linking multiple telescopes together to increase aperture size
What is an impact of atmospheric refraction?
Stars near the horizon appear higher in the sky than they actually are
What discoveries were first made using radio waves?
- Quasars
- The structure and rotation of our galaxy
- Pulsars
- Protoplanetary discs
- Jets from black holes
- SETI (Search for extraterrestrial intelligence)
What discoveries were first made using Infrared (IR)?
- Protostars
- Interstellar dust and molecular clouds
- ‘hotspots’ on the moon
What discoveries were first made using ultraviolet (UV)?
Corona and chromosphere structure of young stars
What discoveries were first made using X-ray?
- Active galaxies
- accretion disks surrounding black holes
- supernova remnants
What discoveries were first made using gamma(γ) rays?
Gamma ray bursts in distance galaxies
Which part of a spectrometer splits up light from astronomical objects into a spectrum?
Diffraction grating
Which types of nebulae expand?
Supernova remnants and Planetary Nebulae
What is the Electromagnetic spectrum from shortest to longest wavelength?
Gamma X-Ray UV Visible IR Microwave Radio
How do we calculate the distance to a star using the HR diagram?
Once you know the absolute magnitude and the spectral type, you can use the distance modulus formula to calculate D. (substituting it in)
How does a short/long period light curve look like?
How would you find the period of an eclipsing binary star?
By charting the brightness of a binary star you can find its period. In the chart on the right, the period would be from the ‘dips’ at 24 and 64 leading to a period of 30 hours.
What is the structure of open clusters?
These can be anywhere from a dozen to thousands of stars making up no particular shape and are found around the galactic plane
What is the structure of globular clusters?
The stars are usually very old red giants and white dwarfs packed tightly together. There are thought to be between 100,000 to over a million stars in a typical globular cluster.
What does a star trail photograph look like?
What is the equation for a star trail calculation?
By looking at how much a prominent star in a star trail photograph has moved we can work out the exposure time of the image.
Most modern astronomical observations…
Are recorded using digital sensors that convert light into electrical signals, which can then be processed and stored as data files
How is light converted into electrical signals?
Light passes through a lens and light is allowed in through a shutter. The light hits a sensor. The sensor (usually a CCD or CMOS*) measures the electrons received and a bayer filter over the CCD measure red, green and blue colours. This is then processed into an image and the information is then saved to a computer file.
Benefits of Earth’s atmosphere:
- Oxygen allows humans and animals to breathe.
- The atmosphere protects us from harmful ultraviolet radiation from the Sun
- Makes the climate and temperature of Earth consistent
- The atmosphere spreads out and scatters light from the Sun
- Protects us from micrometeorite collisions from space
Drawbacks of Earth’s atmosphere:
- The atmosphere holds in some “greenhouse” gases that retain the heat of the Sun.
- Clouds regularly obscure good astronomical viewing
- The atmosphere refracts light which means the position and clarity of star viewing are less accurate
- Pollution from light and chemicals obscure observations
- The atmosphere reflects most radiation making observations at some wavelengths weaker.
What 2 telescopes should only be located sea level of the Earth?
Optical and Radio
Why can Optical and Radio telescopes only be located at sea level?
The rest of the electromagnetic spectrum doesn’t fully get through the Earth’s atmosphere
Why is dry weather important for an observatory location?
Especially important for Infrared astronomy as water vapour in the atmosphere acts as an interference.
Why is a higher location necessary for infrared observations?
Less infrared light penetrates to the surface so a higher location is better for those observations.
Where are larger observatories usually built?
In remote locations far removed from light pollution.
Why are Radio Telescopes typically far away from cities and towns?
As microwaves from mobile phones and other transmitters interfere with signals
How does a telescope alter the appearance of a STAR compared to the Naked eye?
Naked eye:
Appears as dots of light
Telescope:
Sharper dots of light
How does a telescope alter the appearance of a BINARY STAR compared to the Naked eye?
Naked eye:
Appear as a single star
Telescope:
Viewed as multiple stars
How does a telescope alter the appearance of a DOUBLE STAR compared to the Naked eye?
Naked eye:
Just be seen as two stars in dark light
Telescope:
Viewed as multiple stars
How does a telescope alter the appearance of an OPEN CLUSTER compared to the Naked eye?
Naked eye:
Faint stars in close proximity. Some individuals can be seen
Telescope:
Numerous individual stars can be seen
How does a telescope alter the appearance of a GLOBULAR CLUSTER compared to the Naked eye?
Naked eye:
Appear as vague ‘blur’
Telescope:
Detailed numerous stars can be seen
How does a telescope alter the appearance of a NEBULAE compared to the Naked eye?
Naked eye:
Appears as a small but slightly brighter ‘blur’
Telescope:
Shape is more detailed. Colours are more obvious.
How does a telescope alter the appearance of a GALAXY compared to the Naked eye?
Naked eye:
Difficult to view. Andromeda appears as a small dark grey smudge in blur conditions.
Telescope:
Brighter and more detailed view
