Cosmic Vision: Unravelling Rainbows

TL;DR

Color reveals how light and celestial objects behave: astronomers separate light by wavelength to investigate temperature, chemical composition, motion, speed, distance, and dynamics. Katherine Blundell explains rainbow dispersion, sodium’s distinctive orange wavelengths, RGB astrophotography, and spectra produced by spectrographs. Read on to see how astronomers turn color into evidence about otherwise unreachable objects.

Transcript

• Good afternoon. I'm Katherine Blundell Gresham, Professor of Astronomy. The title of my lecture today is Cosmic Vision, Unraveling Rainbows. It can be a beautiful surprise when we get to see a rainbow. I say surprise because seeing a rainbow is not something you can book for or plan, we only see a rainbow when there is a serendipitous distributio... Read More

Key Insights

• 💁 Rainbows are formed by the dispersion of sunlight, revealing different colors due to different wavelengths.
• 💁 Color perception and analysis in astronomy provide valuable information about temperature, chemical composition, speed, distance, and dynamics.
• 🙂 Spectrographs disperse light into spectra, allowing scientists to identify elements and study their properties.
• 🐎 The Doppler Effect in color shifts helps determine the speed and distance of celestial objects.
• ❓ Spectroscopic observations of galaxies enable the measurement of their recession velocity and distance.
• 🤩 Time-resolved spectroscopy reveals the dynamics and motions of objects, such as rotating gas rings or orbiting stars.
• 🫥 Jupiter's spectrum shows dark lines caused by absorption in Jupiter's atmosphere and slanting lines indicating the movement of gases towards or away from Earth.

Questions & Answers

Q: What can color reveal about celestial objects?

Color can reveal a celestial object’s temperature, chemical composition, speed, distance, and dynamics. This is especially valuable in astronomy because researchers cannot directly handle, poke, or prod the objects they study.

Q: How do rainbows separate sunlight into colors?

Raindrops disperse seemingly white sunlight and refract it into cones whose sizes depend on color, or wavelength. Where the cones overlap they produce white light, while their differently sized edges reveal separated colors.

Q: Why are the colors reversed in a secondary rainbow?

The lecture identifies a secondary rainbow whose colors appear in the opposite order from those of the primary rainbow. The excerpt uses this observation to contrast the two rainbows but does not provide a fuller mechanism for the reversal.

Q: What is a glory, and how does it differ from a rainbow?

A glory is not technically a rainbow; it arises through diffraction and back-scattering effects. It can appear when a plane flies above cloud or mist while the sun is low.

Q: How does sodium produce the orange light of street lamps?

Sodium street lamps emit a distinct orange light from two specific wavelengths. Those wavelengths arise from a transition between fixed energy levels in sodium atoms, so the precise orange color indicates sodium vapor.

Q: How do human eyes perceive many colors with only three color receptors?

Human eyes have three color receptors approximately corresponding to red, green, and blue. By interpreting their relative signals, people perceive a much richer spectrum of colors than those three receptor types alone might suggest.

Q: How are color images created in astrophotography?

Astronomers make separate observations through red, green, and blue filters, each passing one part of the spectrum while blocking the others. They color and combine those three images to produce a multicolor image, as demonstrated with the Trifid Nebula, Messier 20.

Q: How do spectrographs help astronomers study celestial objects?

Spectrographs disperse collected light into spectra. The resulting emission lines help astronomers identify chemical elements, while color shifts and time-resolved observations provide information about motion, speed, distance, and dynamics.

Summary & Key Takeaways

• Rainbows are formed by the dispersion of sunlight by raindrops, revealing different colors due to different wavelengths of light.

• Color perception and understanding in astronomy provide important information about temperature, chemical elements, and the dynamics of objects in the night sky.

• Spectrographs disperse light into spectra, allowing scientists to analyze emission lines and identify elements present in celestial objects.