The Beauty and Complexity of Star Birth
This is the image that I constructed of the Orion Nebula (M42)
When we look up at the night sky, we see countless points of light, each representing a distant star. But how did those stars come to be? What is the process by which a cloud of gas and dust transforms into a brilliant, shining ball of plasma, with planets and moons orbiting around it?
One of the best places to look for answers to these questions is the Orion Nebula, a stellar nursery located about 1,344 light-years away from Earth. This nebula is one of the most studied objects in the sky, and for good reason: it is a prime example of an HII region, a vast cloud of ionized hydrogen gas that surrounds a cluster of newly formed stars.
In the later sections, we will explore the properties of the Orion Nebula and how they inform our understanding of star birth. From the density of the gas to the ionizing radiation emitted by the stars, we will uncover the many mysteries of this cosmic laboratory and learn about the ongoing process of stellar creation that is unfolding before our eyes.
The Stellar Nursery: How Stars are Born
Stars are born in vast, cold, and dark molecular clouds that exist in the interstellar space of galaxies. These clouds are composed mostly of molecular hydrogen, with a small fraction of heavier elements, such as carbon, oxygen, and nitrogen, dust particles, and other molecules. Despite their low density of about 100 particles per cubic centimeter, these clouds are enormous, with a size of up to hundreds of light-years across, and can have a mass of millions of times that of the Sun.
The key process in the birth of stars is the gravitational collapse of the gas and dust within these clouds. However, for this collapse to happen, some external force is needed to trigger it, such as a shock wave from a supernova explosion, a collision between two molecular clouds, or the pressure exerted by a nearby massive star. Once this collapse starts, gravity takes over, and the cloud fragments into smaller and smaller clumps, each of which becomes a protostar.
As the protostar continues to accrete material from the surrounding cloud, it begins to heat up, and its core temperature increases. When the temperature reaches about 10 million degrees Celsius, the protostar becomes hot enough to initiate nuclear fusion in its core, converting hydrogen into helium and releasing energy in the process. This energy counteracts the gravitational force, halting the collapse, and stabilizes the protostar. The newborn star has entered the main sequence phase, where it will spend most of its life, steadily burning hydrogen in its core to produce energy and shine bright in the universe.
Star Forming regions
Star-forming regions can take on various forms, depending on the physical conditions and processes that drive their formation. Here are some of the most common types of star-forming regions:
Giant Molecular Clouds (GMCs): GMCs are the largest and most massive star-forming regions in our galaxy, typically spanning tens to hundreds of light-years in size. They are composed of dense, cold gas and dust, primarily molecular hydrogen (H2). GMCs are the birthplaces of most stars, including massive stars that are responsible for ionizing their surroundings and creating HII regions.
HII Regions: HII regions are ionized regions of gas surrounding young, massive stars. They are formed when the intense radiation from these stars ionizes the surrounding gas, stripping electrons from their atoms and leaving behind ionized hydrogen (H+). HII regions can be seen as bright, nebulous patches of gas and are often associated with GMCs.
Stellar Associations: Stellar associations are groups of young stars that formed together from the same GMC. They typically contain a few dozen to a few hundred stars and are often found in the outer regions of GMCs. The stars in stellar associations are usually a few million years old and have dispersed from their birthplace.
Protostellar Cores: Protostellar cores are dense, compact regions within GMCs that are in the process of collapsing to form a single star or multiple stars. They are often detected through their emission in the millimeter and submillimeter wavelengths and are believed to be the precursors to protostars.
Planetary Nebulae: Planetary nebulae are the colorful, glowing shells of gas and dust that are ejected by low-mass stars (like our Sun) at the end of their lives. They are formed when the central star runs out of fuel and sheds its outer layers, exposing its hot core.
These different types of star-forming regions all have their unique characteristics and play a crucial role in the formation and evolution of stars.
Measuring Properties of Star-Forming Regions
Measuring the properties of star-forming regions is essential to understand the physical conditions that lead to the formation and evolution of stars. Astronomers have developed various methods to measure properties such as distance, density, temperature, and ionization rate within these regions.
One of the most important methods to measure the distance to a star-forming region is parallax. Parallax is the apparent shift of an object's position due to the observer's changing point of view. By measuring the angle of the shift and using trigonometry, astronomers can calculate the distance to the object. The Gaia mission, for example, has measured the distances to millions of stars in our galaxy, including many star-forming regions.
To measure the density of gas within a star-forming region, astronomers use different techniques such as radio interferometry, submillimeter observations, and H-alpha emission lines. Radio interferometry is a powerful tool that allows astronomers to see the distribution and density of gas by measuring the interference patterns of radio waves emitted by the gas. Submillimeter observations also provide a way to measure the density of gas by detecting the thermal emission from cold dust grains associated with the gas. H-alpha emission lines, which are produced by ionized hydrogen gas, can also be used to estimate the density of the gas.
The temperature of the gas within a star-forming region can be measured using a variety of techniques, including spectroscopy and photometry. Spectroscopy is a powerful tool that can measure the temperature of the gas by analyzing the emission and absorption lines of different elements in the spectrum. Photometry, on the other hand, measures the brightness of the region at different wavelengths, allowing astronomers to estimate the temperature of the gas based on the shape of the spectrum.
Finally, the ionization rate of a star-forming region can be measured by observing the emission lines of ionized atoms and molecules, such as H-alpha and OIII. The ionization rate is a measure of the number of high-energy photons produced by the hot, young stars in the region. By counting the number of ionizing photons and knowing the properties of the stars, astronomers can estimate the total ionization rate of the region.
Overall, these measurement techniques have allowed astronomers to study the physical properties of star-forming regions in great detail, providing insights into the processes that lead to star formation and evolution.
Constructing the Orion Nebula
The observations were made using the Promt-6 telescope, which is a 50 cm robotic telescope located at Cerro Tololo Inter-American Observatory in Chile. The telescope was equipped with a set of filters to capture light at different wavelengths: Lum (3.0s exposure time), R (8.7s exposure time), V (12.0s exposure time), B (16.0s exposure time), Halpha (75.0s exposure time), SII(Prompt 7) (75.0s exposure time), and OII (75.0s exposure time).
First, I used LRGB filters to capture the broad spectrum of visible light emitted by the nebula. L refers to luminance, which is the total amount of light emitted by the object, while RGB refers to red, green, and blue filters that isolate different parts of the visible spectrum. By combining these images, I was able to create a color image that shows the overall structure and distribution of gas and dust within the nebula.
Image layers consist of Blue (B), Red (R), Green (V), and Luminous (L) respectively.
Image layers consist of Red-Pink (H-α), Blue-Green (OIII), and Red (SII) respectively.
Next, I used narrowband filters to capture specific wavelengths of light emitted by ionized gas within the nebula. In particular, I used filters that isolate the light emitted by hydrogen alpha (Hα), sulfur II (SII), and oxygen III (OIII) lines. When assigning color palettes, I ended up choosing the following: Hα (Balmer), SII(red), and OIII(OIII).
Note also that there were a few different blending modes on the layers. For the RGB layers I used the screen blending mode and the Luminocity mode for the L layer. Finally for the NB filters I ended used the Lighten blending mode (the brightest layer determines the color).
These NB filters are quite helpful as they are produced by different ionization states of hydrogen and other elements within the nebula, and they reveal different physical and chemical conditions of the gas. For example, Hα emission is often used to trace the ionization front of the nebula, while SII and OIII emissions can trace the shock fronts and ionized bubbles created by massive stars.
Unfortunately, I was unable to include some infrared layers within my image since my computer did not want to cooperate with me and download the external data.
The combination of the various filters provided a unique view of the different layers of the Orion Nebula, including the ionized gas, neutral gas, and dust, which allowed us to analyze the physical properties of the region in greater detail.
To further investigate the properties of the Orion Nebula, we performed calculations to determine the density of the HII region and its surroundings. Using the Stromgren-sphere approximation, we calculated the rate at which the central stars emit ionized photons and determined the density of the neutral surrounding gas in the nebula. With these calculations, we estimated the temperature and density of the ionized and neutral gas in the nebula, allowing us to better understand the physical conditions of star formation in this region.
Density calculations
Our next step was to determine the density of the HII region and its surroundings by conducting several calculations. We started by calculating NH+, which represents the rate at which the central stars emit ionized photons using the Stromgren-sphere approximation equation.
To calculate NH+ for the Orion Nebula, I used the Stromgren-sphere approximation equation, which requires determining N* and Rs. To calculate Rs, I used an angular radius of 0.35 degrees, which I found by measuring the field of view in Stellarium. Multiplying this value by pi/180 and then by the distance to the nebula of 410 parsecs, I obtained an Rs value of 3.78 parsecs. Next, I calculated N* by summing up the ionizing flux of the brightest stars in the nebula, resulting in a value of (6.2 x 10^48) ionizing photons per second. Plugging in these values into the Stromgren-sphere approximation equation, I obtained a NH+ value of 154.8 cm^-3.
Using this NH+ value, I then calculated the density of the neutral gas in the surrounding cloud by dividing NH+ by 2, yielding a value of 77.4 cm^-3. To calculate the density of the ionized gas in the HII region, I used the temperature of the neutral gas in the surrounding cloud (5000 K) and the temperature of the ionized gas (10,000 K), along with the NH+ value, to obtain a density of 4,697 cm^-3. This is consistent with the typical range of densities of star-forming regions, which is between 10^2 cm^-3 and 10^4 cm^-3. The density we calculated lies within this range, indicating that it falls within the expected density of the region.
Radio observations
Finally, we analyzed radio observations to detect any bremsstrahlung emissions in the region. Using the Green Bank Observatory telescope and the HI filter, I determined the flux density of Orion A and compared it to that of Taurus A. Unfortunately, my results were not consistent with the expectations given in my density measurements. I figured that this is attributed to the possible error resulting from the calibration observation not being taken immediately after our Orion radio observation.
ACKNOWLEDGMENTS
I would like to thank my Professor, Dr. Reichart, and my TA, Mae Dubay, for the opportunity to capture these amazing images and the opportunity to access the amazing telescopes within the Skynet Network! Also, special thanks to my group mates for all of their help and support throughout this process!
