Constellation is a group of explorers eager to venture forth into the unknown. They're central to the main quest--indeed, most objectives in the campaign come from them. They also have members that will, eventually, become your companions.
How to romance Sarah: Likes and dislikes
The Starfield Sarah romance requires you to boost her affinity by responding in a manner that she likes. Here's a quick summary:

She prefers responses that espouse and praise the work done by Constellation.
General inquisitiveness and support for exploration also provide a boost.

• As a stickler for the rules, Sarah likes it if you resolve conflicts in a peaceful and diplomatic manner.
  
• She does not like underhanded tactics, such as devious deals and plots. However, she wants to see villains get their comeuppance.
  
• As usual, choose the "flirt" reply whenever you see it during a conversation.

In Memoriam quest
• In A Tree Grows in New Atlantis, one of Kelton Frush's associates asks you to tamper with his work background. If you agree to this, Sarah will dislike the response. Immediately following this, you can succeed in a Persuasion minigame to force the NPC to hand over the dataslate, and she'll like that action.
  
• In All That Money Can Buy, there are two prompts where you can let go of a thief named Musgrove. Sarah will dislike both of these responses.
  
• If you plan on joining Ryujin Industries, an NPC named Tomo will pull a gun on you. Diffusing the situation via the Persuasion minigame nets you two likes/approvals.

Meeting Sona and gathering the dogtags

• It's going to be a long trek across ravines and forests, but you'll eventually reach a small hab. Inside, you'll meet a young girl named Sona, the daughter of two of Sarah's comrades.
  
• You'll find a Constellation Guide 03, a skill book that lowers oxygen consumption when you're moving while over-encumbered.
  
• Proceed deeper into the woods until you find a plateau with marked graves. You can collect nine dogtags here.
  
• The tenth and final dogtag is in the belly of a Greater Silverfish, a dinosaur-like creature that you need to kill.
  
• Once you have all the dogtags, return to Sona's hab. You can convince her to leave the place so she can stay with you and Sarah.

Concluding the Sarah romance

• After talking to Sona, exit the hab and speak with Sarah while overlooking the bluffs. Choose: "[Flirt] I'm hoping there's much more than simple gratitude tucked away in that statement."
  
• Return to the Lodge and bring Sarah to the Colony War Memorial in the Embassy District. When prompted, choose: "[Flirt] I only say these things because I care about you, Sarah."
  
• Make your way to the New Atlantis docks and use the elevator near the shops. Select the Waterfall Promenade. There, Sarah will admit that she has feelings for you. This opens up the "Friendship" or "Romance" responses.
  
• Shortly thereafter, Sarah mentions something about her mother and how you might eventually meet her. Respond with: "[Commitment] I'd like to spend our future together, forever."
  
• And, finally, you'll get Sarah's commitment quest. The two of you will get married, with Aja presiding over the ceremony.

he United Colonies were formed after the destruction of Earth's magnetosphere. The billions that have fled the planet managed to find a new home in New Atlantis in Jemison, which became the faction's main base of operations. A central government was formed and, from there, the influence of the United Colonies spread to nearby systems.

A trait, United Colonies Native, grants additional dialogue options and quest rewards, though you'd also end up with a higher bounty on planets controlled by rival factions.
How to join the United Colonies in Starfield
The Starfield United Colonies quest is called Supra Et Ultra. Initially, you'll want to talk to Commander Tuala in the MAST headquarters building in New Atlantis, Jemison. You'll also visit this area as part of an early-game quest, and Tuala himself will ask if you want to become part of the Vanguard, the United Colonies' civilian fleet.
The MAST Building
If you agree to Tuala's proposal, then you're well on your way to joining the United Colonies in Starfield. Here's what you need to do next:

• Use the elevator and go to the Vanguard Orientation Hall.
  
• Confirm the Enlistment Agreement.
  
• You can explore the museum at your leisure, which shows past events in United Colonies history, including the wars against the Va'ruun and Freestar Collective, as well as the emergence of Terrormorphs.

The Simulation Test
When you're done, head to the training area and enter the simulation pod. This test requires you to defeat enemy ships in space, so make sure you use your own spacecraft's weapons and mobility. The goal is to complete three out of six challenge tiers. You can still continue past the third battle to achieve a high score, but that's entirely up to you.
After passing the test, return to Tuala. Follow him to the open grounds in front of the building's entrance. There, he'll ask you to swear allegiance to the United Colonies. Don't worry too much for now since you'll still be allowed to join other factions, such as the Freestar Collective, Ryujin Industries, and Crimson Fleet.

The Freestar Collective was formed out of a desire to break free of United Colonies' rule. As the power of the United Colonies held sway over various planets, there were those who sought more independence and autonomy. These requests were granted, and countless people reached farther across the galaxy.

While the two groups maintained peaceful relations for years, tensions led to an all-out war, which the Freestar Collective eventually won. The faction now guards the frontiers, with their bastion situated in Akila City.

A trait, Freestar Collective Settler, has a similar effect in terms of dialogue options, rewards, and increased bounty on planets owned by other factions.
How to join the Freestar Collective in Starfield
The Starfield Freestar Collective quest is called Job Gone Wrong. You won't have this mission right from the get-go, as you need to head to Akila City. The location is found on the planet of Akila in the Cheyenne system.
The marshal and the Galbank robbery
Once you've arrived, you'll notice a commotion in front of the Galbank building. It seems that some criminals have taken hostages inside the facility. Tell the marshal that you'd like to help, and speak to the robbers via the intercom. If you've put a couple of points into the Persuasion skill, you can attempt to successfully diffuse tensions. Doing so causes the robbers to surrender and some companions, such as Sarah, will like how you handled it. No matter what though, ending the standoff will lead you to the next part of the quest.Emma Wilcox and the Deputized mission
The marshal tells you to talk to Emma Wilcox inside the Rock, the headquarters of the Freestar Collective Rangers. Emma herself isn't too impressed, but she's willing to give you a chance. Your next goal is to check the mission board terminal, where you'll see tasks of the following nature:

• Rescue a captain that's been taken hostage aboard their ship.
  
• Destroy a pirate's ship in orbit over a planet.
  
• Kill an outlaw captain/gang leader in their base of operations.

You only need to complete one objective to be in Emma's good graces. At this point, you and Emma will talk to the marshal. They'll give you your deputy outfit, hat, badge, and the Deadeye revolver.

Take note that, even though you're a member of the Starfield Freestar Collective, you'll still be able to join other factions, such as the United Colonies, Ryujin Industries, and Crimson Fleet.
Montara Luna and The First
The next quest is called Where Hope is Built. The objective is to head to Montara Luna, where you'll talk to farmers that have been harassed by outlaws. Emma will join you as an NPC follower here, not as a full-fledged companion.

To be frank, although we did join the Freestar Collective in Starfield, we did not continue past this stage of the quest arc. That's because we found out that Emma continued to tag along no matter where we went. As such, that made her an extra active follower, leading to a three-person squad.

While governments vied for control in the political arena, Ryujin Industries set about its own conquest in the world of business. After acquiring numerous companies and brands, the conglomerate also opted to engage in a bit of industrial espionage from its headquarters in Neon.

A trait, Neon Street Rat, gives you bonus rewards for quest completions in the city itself.
How to join Ryujin Industries in Starfield
The Starfield Ryujin Industries quest is called Back to the Grind. To start it, you'll need to hear of a rumor concerning their job postings. This can occur as you're exploring New Atlantis, where you'd overhear some NPCs chatting. Alternatively, you can head to the Volii system and the planet of Volii Alpha. There, you'll see a landing site in Neon.
Job application and an interview in Imogene
Upon arriving in Neon Core, look for one of the company's terminals along the street. There should be an option for a job application, which you can fill in. The answers are entirely up to you, though we opted to be more boastful about our character's expertise (i.e. 10+ years of experience). No matter your answers, you'll get invited to a follow-up interview, so don't worry about what you select here.

When you're done, submit your application and make your way to Ryujin Industries' head office on the other side of the thoroughfare. Go up the elevator and talk to Lane. He'll take you to Imogene, the person handling the operations. She'll also ask some typical HR questions, such as how you'd see yourself after five years of working for the company. Answer as you see fit and she'll give you a task.
The coffee errand and Tomo
Your next objective if you want to join the Starfield Ryujin Industries faction is, believe it or not, getting coffee from Terrabrew. Well, you're starting out as an intern, after all.

Make your way to the Terrabrew shop and talk to the barista. Suddenly, a man named Tomo pulls a gun on you. It seems that he used to run errands for Imogene, and he's become somewhat paranoid. Thankfully, you can use your Persuasion skill to calm him down.

After this incident, return to Imogene and tell her about what happened. You'll then go to the boardroom, where you'll talk to various corporate heads, handing them their coffee.
Once you speak with Imogene, she thinks you did a good job, and you'll end up with a promotion. That leads to another objective, where you have to upload software into the databank of a rival corporation. Welcome to the world of corporate espionage and corrupt businesses.

That's all you need to know about how to join Ryujin Industries in Starfield. Don't forget that you can still be part of the United Colonies Vanguard, Freestar Collective Rangers, and the Crimson Fleet.

The House of the Enlightened is one of the religious factions in Starfield. Think of them as atheism on a galactic scale. They do not believe in a divine being, but they aim to provide help to those in need on the basis of human kindness and empathy, as opposed to religious dogma. The Raised Enlightened trait nets you additional dialogue options, as well as a chest in the sect's main building in New Atlantis' The Well area.

The Sanctum Universum is an amalgamation of monotheistic faiths. Diametrically opposed to the Enlightened, those who follow the path of the Universal believe in the capital-G God, one who has guided humanity's journey across the stars. The Raised Universal trait has a similar effect, and you'll find their church is in the Embassy District in New Atlantis.

Compared to the two religious sects mentioned above, the Va'ruun are completely hostile and relentless. They believe in only one thing: the coming of the Great Serpent and the annihilation of those who do not follow the path. Eschewing all trappings of a normal life on a planet, their forces prefer life in spacecraft, endlessly traversing the emptiness of the void.

The Great Serpent trait doesn't give you an extra chest. Rather, it grants boosts after grav jumping to another star system, and penalties if you don't do grav jumps for prolonged periods. As of the time of this writing, we've yet to find out if it's possible to join the faction.

The Crimson Fleet is the largest warband of space pirates, striking terror into the hearts of hapless merchantmen and civilians. Although the United Colonies and Freestar Collective try their best to guard their respective systems, the pirates still manage to get away. Still, you might find yourself infiltrating the group, as tasked by the UC SysDef operations.
How to join the Crimson Fleet in Starfield
The Starfield Crimson Fleet quest is called Deep Cover. It's not available at the start of the game. Rather, you'll only receive it if you commit a crime in a system controlled by the United Colonies. Here's a quick summary:

• If you get caught stealing or assaulting someone in New Jemison, a guard will approach you.
  
• Alternatively, if contraband is detected on your ship, you'll get detained.
  
• There's also a third method, which is to simply talk to the United Colonies SysDef personnel.

United Colonies SysDef and an offer you can't refuse
Regardless of your decision, you'll be well on your way to joining the Crimson Fleet in Starfield once you meet up with Commander Ikande. He has an offer you can't refuse: go to jail for your crime, or find a way to infiltrate the Crimson Fleet. Naturally, you'll want to choose the latter.

Since you're undercover, Ikande tells you that the pirates might ask you to commit crimes and atrocities. The choice of how to proceed in those situations is entirely up to you. However, if you're caught by United Colonies forces, you'll still need to pay fines.
Deep Cover: Cydonia
The next task for the Starfield Crimson Fleet takes you to Cydonia on Mars. Here's a quick summary:

• Talk to a corrupt Trade Authority officer named Saoirse. Give her 1,000 credits so she tells you more about a shipment of Aurora, a drug that's being moved around in the Settled Systems.
  
• In the Cydonia bar, give the sample to Adler Kemp, and say that he might be able to help you "move cargo."
  
• Adler wants you to collect the dues from a fella named Karl, who's in the Deimos Miner's Quarters. You can either pay his debts (3,000 credits), pass a Persuasion check, or attack him outright.
  
• Return to Adler and he'll mention a ship called the Astraea. This leads to the next quest called Rook Meets King.

Rook Meets King: Naeva, the Astraea, and Austin Rake
The Starfield Rook Meets King quest involves Naeva, a high-ranking member of the Crimson Fleet. Here's the gist:

• Fly to Europa, where you'll find the Astraea ship orbiting the moon. Hail the ship and talk to Naeva, who mentions someone named Austin Rake, a former member who has since escaped.
  
• Head to Enceladus and hail the Ragana, a civilian transport that's hiding Austin Rake. You're presented with several options here:
  
• UC SysDef: You can board the ship to talk to the captain and crew.
  
• Crimson Fleet: You'll still be able to board the ship, though the demand is quite intimidating.
  
• You can tell the crew to kill Austin Rake.
  
• You can immediately open fire to destroy the ship.
  
• If you boarded the Ragana, you'll be able to talk to the captain. This gives the option to let Austin Rake go (he'll stay in hiding and there will be no hostilities). Alternatively, you can attack and kill everyone.
  

When you're finished with these tasks, return to Europa and talk to Naeva once more. In our playthrough, we chose the UC SysDef reply, and we let Austin Rake escape. As such, we were able to provide this reply to Naeva: "[Lie] Austin Rake is dead. The Ragana survived."
The Lock and The Key
After completing the above objectives, Naeva will give you the coordinates to The Key, a massive star station that houses the bulk of the Crimson Fleet's forces. You can find it orbiting the planet of Suvorov in the Kryx system.

Upon docking, there's a short scene involving two pirates. Naeva will then greet you, taking you on a tour of the base and introducing you to various NPCs. At the end of it all, you'll meet the head honcho, Delgado. You're a member of the Starfield Crimson Fleet faction now, but he still wants you to head to the prison complex known as The Lock, sort of a way to prove yourself.
Additional tips when joining the Crimson Fleet

• UC SysDef has another major mission for you called Burden of Proof. This goes hand-in-hand with the other objectives, since the goal is to recover documents and data slates regarding the pirate group's activities. You can bring any evidence you find to Lieutenant Jillian Toft.
  
• The Crimson Fleet has its own Trade Authority vendor, which means you can sell contraband and stolen items. Even better, The Key space station doesn't conduct a scan--that would make no sense since they're lawless vagabonds.
  
• There are quests that give you alternative options and responses (i.e. UC SysDef or Crimson Fleet), as well as actions that are more nefarious (you're working with pirates, after all). Bear in mind that getting caught committing these crimes still nets you a bounty. Moreover, your good-natured companions (i.e. everyone) will not approve of evil actions, too.
  
• By default, Crimson Fleet vessels will attack you on sight. However, once you become a pirate newbie, they'll turn into friendlies. As such, we don't recommend joining until much later in the game, since you'll end up with fewer ships for target practice and XP gains.
  
• Conversely, Crimson Fleet ships and NPCs might be designated as targets for certain quests or mission board tasks. They'll appear as friendlies if you've already joined the faction, though you'd still be able to attack them. As far as we can tell, this did not cause further hostilities with the faction, though we can't confirm if this will be the case for every quest.

The other mercenary groups--Spacers, Ecliptic, and The First--are encountered as you explore various planets and star systems. There are instances where you'd grav jump to another location, only to realize that their engines are burning and their weapons are primed.

Likewise, due to the cessation of hostilities between the United Colonies and Freestar Collective, numerous bases and outposts have been left abandoned, ready for the taking. As such, you might even see these troops defending those facilities on various planets.

Starfield factions also include those that you encounter on planets and ships. As far as we can tell, they do not have their own lengthy quest chain. Instead, they might have side quests posted on mission boards in cities, or they'd have their own ships plying trade routes. Here are some examples:

The Trade Authority has job postings for cargo delivery and passenger ferrying.
Trackers may ask you to eliminate a target and claim the bounty on their head.
Galbank has branches in cities, but the company also has armored ships loaded with credits.
Stroud-Ecklund is a corporation that builds ships, and you might be asked to help their R&D section in the Overdesigned mission.