Astrophysics Flashcards
(73 cards)
What is a real image?
A visual representation of an object, formed where scattered light from a point on the object reconverges, on the opposite side of the lens (as the object), and is always inverted
e.g. the image formed by an eye on the retina
What is the condition for the formation of a real image?
Object must be further away from the lens than the focal point
What is a virtual image?
A visual representation of an object, formed where scattered light from a point on the object appears to converge, on the same side of the lens (as the object), and is not inverted
e.g. the image formed when looking through a magnifying glass
What is the condition for the formation of a virtual image?
Object must be nearer to the lens than the focal point
What is magnification?
The ratio of the angle subtended by the image to the angle subtended by the object
What do refracting telescopes use to focus light?
Two converging lenses: the objective lens and the eyepiece
What does normal adjustment in a telescope mean?
The final image is at infinity; parallel rays entering the telescope result in parallel rays emerging from the eyepiece
What is chromatic aberration?
Distortion in an image, which occurs because different wavelengths get refracted by different amounts
How do reflecting telescopes avoid chromatic aberration?
By using mirrors instead of lenses to focus light
What is spherical aberration?
Distortion in an image, which occurs when using spherical mirrors/lenses which do not have a fixed focal point
What type of mirrors/lenses must be used to avoid spherical aberration?
Parabolic
What is collecting power in telescopes?
The rate at which useful energy is received, which is a function of the area of the collecting surface
What is resolving power?
The smallest angle at which a telescope can accurately discern two separate points
What does the Rayleigh criterion state?
The minimum separation for two points to be clearly resolved occurs when the central maximum of one overlaps the first minimum of the second
What are Charge-Coupled Devices (CCDs)?
Sensitive light detectors made of semiconductor layers that capture photons
What is quantum efficiency in the context of CCDs?
The percentage of incident photons detected by the CCD (for a given wavelength)
How does the quantum efficiency of CCDs compare to the human eye?
CCDs can have a quantum efficiency of about 70% to 95%, while the human eye is about 1%
What is a proto-star?
A dense ball of dust in space that heats up and begins to glow
What happens to a protostar when the temperature at the core reaches ~15 million Kelvins?
It begins to fuse hydrogen (at the core), and becomes a main sequence star
What occurs when hydrogen fusion stops in the core of a star?
The core collapses, becoming denser and hotter, and in some cases, this can causes the start of Helium fusion
What is a red giant?
A star that has left the main sequence and greatly expanded due to hydrogen fusion occurring in it’s core’s outer layers
What happens to a star after all the helium is fused?
It may continue fusing heavier elements until it reaches iron, or it may cease fusion
What happens when fusion stops in smaller red giants (less than 8 solar masses)?
They eject their outer layers, creating a planetary nebula and leaving a white dwarf
What is a white dwarf?
A small, dense, extremely hot core left after a star ejects its outer layers
