Astronomy Lecture Notes: Infrared, Telescopes, Twinkling, and Atmospheric Optics
Infrared Camera and Thermal Radiation
Infrared Camera Functionality: Measures heat radiation and transforms it into a visible light palette.
Color Mapping: Colder objects (e.g., chairs) appear blue, while warmer objects (e.g., human bodies) appear yellow, orange, or red. This mapping aligns with common human perception of temperature (blue for cold like glaciers, red for hot like coals).
Natural Emission: Our natural body emission is in the infrared spectrum, which the camera detects.
Material Transparency: Some materials are opaque to visible light but transparent to infrared, and vice versa.
Example 1: A dark bag that is opaque to visible wavelengths (blocking facial features) becomes transparent to infrared, revealing facial features.
Example 2: A piece of lucite is transparent to visible light but opaque to infrared.
Glasses: Glasses often appear blue in infrared images because they are largely opaque to infrared and can be colder than the face due to imperfect heat transfer.
Temperature Observations: Laptops, despite generating heat, often appear cold on their backsides.
Demonstrations for Office Hours:
Heating a rod with a torch to observe slow heat diffusion.
Painting initials/shapes on skin with ice, which appears as a blue mark on otherwise warm, red/orange/yellow skin.
Practical Applications: Infrared devices can detect heat leaks in houses.
Temperature Scale: The camera's color scale adjusts dynamically based on the objects being observed.
Default Scale: Blue might represent 20.5^ ext{o} ext{C}, while red/white represents 35^ ext{o} ext{C}.
Readjustment: Pointing at a hot lamp can cause the scale to readjust, making white represent almost 100^ ext{o} ext{C} and blue represent 18.6^ ext{o} ext{C}.
Course Logistics and Announcements
Today's Topics: Finishing telescopes, discussing star twinkling, and lunar phases.
Office Hours: Very well attended; students are encouraged to continue coming (today 10-11 AM for infrared camera play, also later this week for pickle experiment follow-up).
Pickle Experiment: Revisited from last Friday, where interesting patterns regarding which side of the pickle turned on were observed but need further verification. More pickles will be acquired.
Double Feature Lectures: Two lectures today (one currently, another 4-5 PM); both will be recorded.
Star Parties: Scheduled for tonight and tomorrow night, weather permitting; available throughout the semester.
Campbell Hall Access: Unofficial suggestion that the back door of Campbell Hall might be unlocked at 6:00 PM, allowing access to the sixth floor stairs. Ethical implications of propping doors open (lobby door is a clear violation, sixth-floor door less so in terms of general public access). Speaker takes responsibility for this suggestion.
Homework: Homework 3 is due this Friday.
Talks: Scheduled for Wednesday and Thursday.
Reflecting Telescopes and Obstruction
Reflecting Telescope Design: Consist of a primary mirror and a secondary mirror.
Secondary Mirror Function: Deflects light to the eyepiece, either at the back of the primary mirror or off to the side (as Isaac Newton designed).
Purpose: Prevents the observer's head from blocking incoming light at the prime focus, which was significant for smaller, older telescopes.
Image Formation: Every part of the mirror forms a complete image; blocking a portion of the mirror does not create a "hole" in the observed image (e.g., Jupiter would not have a hole).
Parallel Rays: The reason blocking part of a camera lens with a finger creates a visible obstruction is due to the close proximity of the object, meaning the light rays are not parallel, unlike light from distant astronomical objects for a telescope.
Landmark Telescopes and Role of AI
Speaker's Experience: The speaker has used several notable telescopes throughout their career, including:
Palomar 200-inch telescope (for doctoral thesis).
Mount Wilson 100-inch (2.5 meter) telescope (once the world's largest).
Lick Observatory 3-meter telescope (used for 40 years).
A completely robotic small telescope at Lick Observatory for searching and following up on exploding stars.
AI in Robotics: Artificial Intelligence has been used in robotics for decades, predating the recent widespread use of tools like ChatGPT and Gemini, indicating that various forms of AI and machine learning have a long history.
Mauna Kea Observatory, Hawaii: A significant observatory for over half a century.
Keck Telescopes: Two 10-meter optical telescopes, considered the largest in the world in terms of collecting area.
Comparison: While another telescope in the Canary Islands is 0.4 meters larger in diameter, the Keck telescopes have a larger collecting area due to a smaller secondary mirror obstruction.
Segmented Mirror Design: Each 10-meter Keck mirror consists of 36 hexagonal segments, which function as a single continuous mirror through precise alignment maintained by actuators.
James Webb Space Telescope (JWST): Its design is patterned on this segmented mirror technology.
Research: The speaker's research group has scheduled time on Keck 2 for two half-nights, operating remotely from Berkeley (3 hours ahead of Hawaii time).
Radio Telescopes: Size and Resolution
Angular Resolution (Clarity): To achieve the same angular resolution as an optical telescope, a radio telescope must be "much larger" than an optical telescope.
Principle: Clarity is inversely proportional to the telescope's diameter and directly proportional to the wavelength of light being observed. The formula for angular resolution is approximately heta imes rac{ ext{wavelength}}{ ext{diameter}} or heta imes rac{ ext{λ}}{ ext{D}}.
Reasoning: Radio wavelengths are significantly longer (e.g., meters to kilometers) than optical wavelengths (nanometers). To compensate for a large wavelength ( ext{λ}) and achieve a small blur circle angle ( heta), the telescope's diameter ( ext{D}) must be very large.
Examples of Radio Telescopes:
Owens Valley: Run by Caltech, located between the Sierras and White Mountains.
Arecibo (Puerto Rico): A 300-meter diameter radio dish that operated for decades but collapsed five years ago. Its site is being restored to its natural jungle state.
Combined Arrays: Multiple radio telescopes can be combined to achieve high resolution.
Very Large Array (New Mexico): Consists of 27 radio dishes.
Principle: Such arrays do not have the collecting area of a single dish of equivalent diameter, but they achieve the resolution (clarity) of a single dish with a diameter equal to the maximum separation between the farthest dishes. This is because the large separation provides a large effective D in the resolution formula, although operating them in this way is technically challenging.
Advantages of Space Telescopes
Higher Cost: Launching telescopes into space is significantly more expensive than ground-based options.
Primary Advantages:
Elimination of Atmospheric Distortion: Earth's atmosphere blurs images of distant stars. In space, stars appear as sharp points of light, improving clarity.
Darker Sky:
Light Pollution: Ground-based observatories, like Lick Observatory, suffer from increasing light pollution from nearby cities (e.g., San Jose), limiting the ability to observe faint, distant objects. Space offers a truly dark sky.
Atmospheric Glow in Infrared: Even away from cities, Earth's warm atmosphere glows brightly in the infrared spectrum due to jiggling particles, making the long infrared night sky as bright as the daytime sky. Space avoids this thermal glow.
Access to Blocked Wavelengths: Earth's atmosphere blocks or strongly absorbs many wavelengths of the electromagnetic spectrum.
Infrared (IR): Many IR wavelengths are blocked by atmospheric molecules and water vapor, necessitating high-altitude ground observatories or space telescopes.
Ultraviolet (UV): Most UV light is blocked by the ozone ( ext{O}_3) layer, which protects life but hinders UV astronomy.
X-rays and Gamma Rays: Nearly all X-rays and gamma rays are blocked by atmospheric molecules, preventing harmful radiation from reaching the surface but requiring space-based instruments for observation.
Optical and Radio: Most optical and some radio wavelengths reach the ground, allowing for ground-based telescopes in these regimes.
Specific Space Telescopes and Their Contributions
Swift Observatory: Primarily for observing gamma-ray bursts. The speaker highlights the strict deadlines for grant proposals for such observatories, contrasting them with flexible academic deadlines.
Spitzer Space Telescope: An infrared observatory that provided valuable data.
Chandra X-ray Observatory: An X-ray observatory that is still operating, though facing funding cuts.
Hubble Space Telescope (HST):
Launch and Orbit: Launched in 1990, it operates in low Earth orbit (around 340 miles above the surface).
Naming: Named after Edwin Hubble, who first demonstrated external galaxies and played a key role in discovering the expansion of the universe.
Mirror Precision: Features a 2.4-meter diameter mirror that is incredibly smooth; if expanded to the size of the US, irregularities would be only a few inches, compared to skyscrapers for typical glasses.
Initial Flaw: The primary mirror had an incorrect overall shape, suffering from spherical aberration, causing blurred images instead of a sharp focus.
NASA's Error: Conflicting lab tests regarding the mirror's shape were ignored, and the telescope was launched prematurely.
Repair Mission: In 1993, astronauts installed corrective optics (like "contact lenses" using mirrors to avoid chromatic aberration) during a servicing mission, successfully fixing the flaw.
Impact: Though an initial embarrassment and costly, the refurbishment was successful, leading to clearer images and significant scientific contributions.
Iconic Images: Famous for iconic photos, such as the "Pillars of Creation" (a stellar nursery), which, despite being stunning, led to incorrect viral headlines.
James Webb Space Telescope (JWST):
Launch and Location: Launched on Christmas Day 2021, positioned at the second Lagrangian point (L2), approximately four times the distance to the Moon ($\approx 1.5 imes 10^6 km from Earth in the anti-solar direction).
Orbit: Orbits the Sun in one year, maintaining approximate collinearity with Earth.
Wavelength Range: Tuned to infrared (heat radiation), making it complementary to Hubble (which focuses on ultraviolet and visible light), rather than a successor.
Why Infrared?:
Probing Different Processes: Different physical processes emit radiation in distinct parts of the electromagnetic spectrum.
Observing Colder Objects: Detects radiation from cooler objects like smaller stars, brown dwarfs, and planets that do not emit strongly in visible light.
Seeing Through Dust: Infrared light can penetrate clouds of gas and dust (e.g., stellar nurseries like the Pillars of Creation), revealing newly formed stars hidden from optical view.
Detecting Distant Objects (Redshift): The expansion of the universe stretches the ultraviolet and optical light from very distant, early galaxies into infrared wavelengths (a cosmological redshift, distinct from the Doppler effect).
Thermal Control: Maintained at extremely cold temperatures by a five-layer sunshield, the size of a tennis court. It creates a temperature differential of 650^ ext{o} ext{F} (from 260^ ext{o} ext{F} on the sunlit side to --390^ ext{o} ext{F} behind the shields).
Mirror Design: Features 18 hexagonal mirror segments, based on the Keck design.
Diffraction Spikes: Bright stars in JWST images show characteristic six-point diffraction spikes, caused by light interacting with the mirror edges. These are a hardware effect that must be removed via software if an unblemished star image is desired.
Initial Alignment: Upon deployment, the 18 segments initially produced 18 separate images of a star. Actuator adjustments were made to align them into a single, focused image.
Early Discoveries: The first images from JWST (March 2022, official release July 2022) immediately led to significant findings, including:
Detection of faint, red galaxies formed much earlier than expected (a few hundred million to less than 1 billion years after the Big Bang).
Observations of gravitational lensing (curved arcs of background galaxies distorted by foreground massive objects).
Estimates of the total number of galaxies in the observable universe (trillions, based on the "grain of sand at arm's length" analogy for the field of view).
Twinkling of Stars (Scintillation)
Phenomenon: Stars appear to twinkle or scintillate, but this is an atmospheric effect, not an intrinsic change in stellar brightness.
Cause: Earth's atmosphere is not uniform; it contains turbulent pockets of air with varying density, humidity, and temperature.
Refractive Index Differences: These variations cause differences in the atmosphere's refractive index.
Light Bending: Light rays from a star are bent by different amounts and in varying directions as they pass through these constantly moving turbulent pockets.
Eye's Perception: Sometimes many bent rays strike the eye simultaneously (making the star appear brighter), while at other times, fewer rays do (making it appear fainter).
Analogy: Similar to looking at the bottom of a swimming pool with ripples on the surface. The ripples focus and defocus sunlight, creating bright and dark patches that appear to move.
Effect of Horizon: Stars twinkle more intensely closer to the horizon because observers are looking through a greater thickness of atmosphere and more independent turbulent patches.
Distinguishing Stars from Planets by Twinkling
Practical Observation: Planets generally twinkle less than stars when observed at similar heights above the horizon.
Reasoning:
Stars: Due to their immense distance, stars appear as virtual point sources of light (even through telescopes). The light from this single point is acutely affected by atmospheric turbulence.
Planets: Planets are much closer and appear as resolved disks (e.g., Jupiter's disk, Saturn's rings) even through small telescopes. They are effectively a collection of many points of light.
Averaging Effect: While different parts of a planet's disk may twinkle independently, the overall light from the entire disk averages out, resulting in much steadier apparent brightness compared to a single point-like star.
Exception: Near the horizon, planets will also twinkle significantly due to the extensive amount of atmosphere their light must traverse.
Blue Sky and Red Sunsets
Atmosphere Thickness: Earth's radius is approximately 6,400 km, with the effective atmosphere extending about 100 km (most weather in the first 10 km).
Path Length: Looking straight up means light passes through approximately 1 "atmosphere" of thickness. Looking towards the horizon means light passes through a significantly greater thickness of atmosphere and more turbulent air packets.
Blue Sky Explanation:
Rayleigh Scattering: Blue light (shorter wavelengths) is preferentially scattered by air molecules more effectively than longer wavelengths (green, yellow, orange, red).
Sun's Spectrum and Eye Sensitivity: The sun emits a broad spectrum of light. While it emits violet, our eyes are less sensitive to it, and the sun emits less violet. The sun emits a lot of blue light, and our eyes are very sensitive to blue. This combination leads to the preferential scattering of blue light.
Result: When looking away from the sun, the scattered blue light dominates, making the sky appear blue.
Sun's True Color: The Sun is intrinsically a white star, not yellow, although children's books often depict it as yellow.
Definition of White Light: The combination of all colors of the rainbow in the proportions emitted by the sun is perceived as white light.
Evidence: The sun appears white on foggy days (without filters), during solar eclipses, and cumulus clouds, snow, and the full moon (which reflect unfiltered sunlight) all appear white.
Yellow/Orange/Red Sunsets and Sunrises:
Increased Atmospheric Path: As the sun approaches the horizon, its light travels through a much greater amount of atmosphere.
Scattering of Shorter Wavelengths: This longer path provides more opportunities for atmospheric molecules to scatter out the shorter wavelengths (violets, blues, greens) from the direct path of sunlight.
Remaining Light: The remaining light that reaches our eyes is predominantly the longer wavelengths: yellows, oranges, and reds. The closer to the horizon, the more pronounced this effect, leading to deeper reds.
Role of Particulate Matter: Dust from storms or smoke from wildfires (e.g., California fires, Burning Man) enhances this effect, as these particles also preferentially scatter and absorb shorter wavelengths.
Example: Photos from Helsinki show the sun progressing from yellow to red as it nears the horizon due to increased particulate matter and atmospheric path.
Intense Red: During wildfires, the sun can appear blood-red even when significantly above the horizon.
Colored Clouds: Clouds at sunrise or sunset appear yellow, orange, or red because they receive and reflect this preferentially scattered, longer-wavelength light from the sun. The specific colors depend on cloud height, composition, and particle content.